<PAGE>
 
                                                                   Exhibit 10.43
                                                                  EXECUTION COPY

                                  ASSIGNMENT
                                  ----------


          THIS ASSIGNMENT has an effective date of May 19, 1998 (the "Effective
Date") and is made between MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 77
Massachusetts Avenue, Cambridge, MA 02139 ("Licensor"), SEPRACOR INC., 33 Locke
Drive, Marlborough, MA 01752 ("Sepracor") and CHIREX AMERICA INC., 300 Atlantic
Street, Suite 402, Stamford, Connecticut 06901 ("ChiRex").

          WHEREAS, Licensor and Sepracor are parties to a License Agreement,
dated May 5, 1989 amended on March 24, 1995, June 22, 1995, June 26, 1995 and by
a Letter of Understanding dated May 20, 1996 (collectively referred to herein as
the "License Agreement") covering MIT Case Number 4585, "Catalytic Asymmetric
Dihydroxylation of Olefins" by Wilhelm K. Amberg, Declan G. Gilheany, Byeong M.
Kim, Hoi-lun Kwong, Istvan Marko, K. Barry Sharpless, Tomoyuki Shibata, John S.
Wai and Lisa Wang, and MIT Case Number 4619, "Synthesis and Applications of
Tartrate Cyclic Sulfates", by Yun Gao and K. Barry Sharpless; and

          WHEREAS, ChiRex has practiced the Patent Rights under the License
Agreement as a sublicensee of Sepracor under a Technology Transfer and License
Agreement between Sepracor and SepraChem (a predecessor of ChiRex) effective
January 1, 1995 (the "Technology Agreement") and the aforementioned Letter of
Understanding; and

          WHEREAS, since the merger of Sepracor's Seprachem subsidiary into
ChiRex, ChiRex has exploited the Patent Rights to a greater extent than Sepracor
and the parties to this Amendment Agreement have agreed that Sepracor shall
assign the License Agreement to ChiRex, and ChiRex shall, from the Effective
Date, become the Licensee under such agreement pursuant to the terms set forth
herein; and

          WHEREAS, Licensor has also agreed that ChiRex may grant Sepracor a
sublicense under the Patent Rights pursuant to Section 2.7 of the License
Agreement.

          NOW THEREFORE, in consideration of the premises and the mutual
covenants contained herein, the parties hereto agree as follows:

          1.   Assignment.  Sepracor hereby assigns its rights and obligations
               ----------                                                     
under the License Agreement to ChiRex, subject to Sepracor's rights to practice
the Patent Rights pursuant to its sublicense per Section 3 of this Assignment.
Licensor hereby consents to the assignment of Sepracor's rights and obligations
to ChiRex, and ChiRex agrees to abide by the terms of the License Agreement and
assume all of Sepracor's obligations under said agreement as of the Effective
Date. The term Licensee as defined in the License Agreement shall hereinafter
mean ChiRex.

                                       1
<PAGE>
 
          2.   Notices.  Article XIV of the License Agreement shall be amended
               -------                                                        
so that notices to Licensee shall be sent to:

               ChiRex America Inc.
               300 Atlantic Street, Suite 402
               Stamford, CT 06901
               Attention: General Counsel
               Telephone: (203) 351-2300
               Fax: (203) 425-9996

          3.   Sublicense.
               ---------- 

          (a)  ChiRex hereby grants Sepracor a worldwide, exclusive (even as to
          ---                                                                  
               ChiRex) sublicense under the Patent Rights, to develop, make, use
               and sell (i) those compounds included in Exhibit C-1 attached
               hereto and made a part hereof; (ii) Combinatorial Chemistry
               Libraries (as defined below) of chiral or achiral compounds; and
               (c) compounds in the Combinatorial Chemistry Libraries in
               quantities of less than one (1) kilogram. In addition, ChiRex
               further grants to Sepracor a worldwide non-exclusive sublicense
               under the Patent Rights to manufacture pharmaceutical fine
               chemical intermediates and pharmaceutical active ingredients for
               the clinical and laboratory use of Sepracor and its licensees.
               Such sublicenses shall bear the terms and conditions as set forth
               in Articles II, V, VII, VIII, IX, X, XII and XV of the and
               Sepracor hereby represents and warrants that it has a copy of the
               License Agreement, is familiar with its terms and will comply
               with such terms. Notwithstanding Section 4.1(d) of the License
               Agreement, Licensor will receive the same royalties on Net Sales
               of Licensed Products by Sepracor as it would receive on Net Sales
               of Licensed Products by ChiRex.

          (b)  The sublicense granted per subsection (a) above shall include the
          ---                                                                   
               right to sublicense (within the scope of the rights granted
               therein) third parties without ChiRex's consent provided (i) all
               sublicenses granted by Sepracor will be in writing,  charge
               sublicensees royalties per Section 4.1(d) of the License
               Agreement, and expressly require all sublicensees to comply with
               the terms of the License Agreement; (ii) Sepracor provides
               Licensor and ChiRex with notice of and a copy of all written
               sublicenses entered into; and (iii) Sepracor and not ChiRex,
               shall remain responsible for the performance, collection and
               remission of royalties by all its sublicensees.

          (c)  Sepracor shall have the right to notify ChiRex in writing, at any
          ---                                                                   
               time after Sepracor or one of its licensees begins clinical
               trials, that it wishes to add to Exhibit C-1 any (i) active
               metabolite compound; or (ii) single isomer pharmaceutical
               compound; or 

                                       2
<PAGE>
 
               (iii) other chiral compound which is identified as being from a
               Combinatorial Chemistry Library; such compounds shall be added to
               Exhibit C-1 subject to the procedures and terms set out in
               Exhibit C of the Technology Agreement.

          (d)  As used herein, "Combinatorial Chemistry Library" shall mean any
          ---                                                                  
               group of 25 or more compounds related in structure and
               synthesized contemporaneously from a common intermediate in
               quantities of no more than one hundred grams per compound.

          (e)  Licensor hereby consents to the grant of the sublicense to
          ---                                                            
               Sepracor specified herein.

          4.   ChiRex Termination.  Should ChiRex abandon, breach or otherwise
               ------------------                                             
fail to maintain the License Agreement, provided Sepracor is not in breach of
the terms of its sublicense, Licensor agrees to grant Sepracor a direct license
on the same terms and conditions set forth in Section 3 so that Sepracor's use
of the Patent Rights is uninterrupted.

          5.   No Other Changes.  Except as is expressly stated in this
               ----------------                                        
Assignment, the terms of the License Agreement remain in full force and effect.

          6.   Definitions.  Unless defined specifically herein, all capitalized
               -----------                                                      
terms used in this Assignment shall have the meaning ascribed to such terms in
the License Agreement.

          7.   Counterparts.  This Assignment may be executed in one or more
               ------------                                                 
counterparts, each of which shall be deemed an original, but all of which
together will constitute one and the same instrument.

                                 * * * * * * *

                                       3
<PAGE>
 
          IN WITNESS WHEREOF, the parties have caused their duly authorized
representative to execute this Agreement as of the Effective Date.

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

BY:_______________________
NAME:_____________________
TITLE:____________________



SEPRACOR INC.                            CHIREX AMERICA INC.

BY:_______________________               BY:______________________
NAME:_____________________                   NAME:_____________________
TITLE:____________________                   TITLE:____________________

                                       4
<PAGE>
 
                                  Exhibit C-1
                                  -----------

Terfenadine carboxylate (racemate and single isomers)
R-Ketoprofen
S-Ketoprofen (for use in dentifrice or mouthwash formulations)
R-Albuterol
R,R,-Formoterol
R-Fluoxetine
S-Fluoxetine
S-Oxybutynin
R-Onybutynin
S-Doxazosin
Norastemizole
Norcisapride (racemate and single isomers)
R-Ondansetron
S-Ondansetron
(-)-Amlodipine
Pantoprazole single isomers
Ketoconazole single isomers
Itraconazole single isomers
Descarboethoxyloratadine
Lomefloxacin single isomers
Ketorolac single isomers
Etodolac single isomers
Metoprolol single isomers
Cisapride (racemate and single isomers)
Salmeterol single isomers
Zoplicone single isomers
Sibutramine single isomers
Cetirizine single isomers
Zileuton single isomers
Hydroxyitraconazole single isomers (other than racemic
     hydroxyitraconazole)
Glycopyrrolate single isomers
Clidinium single isomers
Tridihexethyl single isomers
Trihexplenidyl single isomers
Desformoterol single isomers
Desethyloxybutynin single isomers
Procyclidine single isomers
Lansoprazole single isomers
Bupropion single isomers
Rabeprazole single isomers
Hydroxyomeprazole single isomers
Omeprazole single isomers

                                       5
<PAGE>
 
                                                                           10.43
                                                                  EXECUTION COPY
                                  ASSIGNMENT
                                  ----------


          THIS ASSIGNMENT has an effective date of May 19, 1998 (the "Effective
Date") and is made between RESEARCH CORPORATION TECHNOLOGIES, INC., 101 N.
Wilmot Road, Suite 600, Tuscon, Arizona 85711-3355 ("Licensor"), SEPRACOR INC.,
33 Locke Drive, Marlborough, Massachusetts 01752 ("Sepracor") and CHIREX AMERICA
INC., 300 Atlantic Street, Suite 402, Stamford, Connecticut 06901 ("ChiRex").

          WHEREAS, Licensor and Sepracor are parties to a License Agreement,
dated September 10, 1992, as amended pursuant to a First Amendment dated
September 10, 1992, a Second Amendment dated January 1, 1995, a Third Amendment
dated March 5, 1996 and a Fourth Amendment dated November 20, 1996 (the License
Agreement, First Amendment, Second Amendment, Third Amendment and Fourth
Amendment collectively referred to herein as the "Jacobsen License Agreement");

          WHEREAS, pursuant to the Third Amendment, ChiRex has been considered
an "Affiliate" of Sepracor under the Jacobsen License Agreement and has
practiced the Licensed Patents thereunder pursuant to the Third Amendment as
well as under a Technology Transfer and License Agreement between Sepracor and
SepraChem (a predecessor of ChiRex) effective January 1, 1995 (the "Technology
Agreement"); and:

          WHEREAS, since the merger of Sepracor's Seprachem subsidiary into
ChiRex, ChiRex has exploited the Licensed Patents to a greater extent than
Sepracor and the parties to this Amendment Agreement have agreed that Sepracor
shall assign the Jacobsen License Agreement to ChiRex, and ChiRex shall, from
the Effective Date, become the Licensee under such agreement pursuant to the
terms set forth herein; and

          WHEREAS, Licensor has also agreed that ChiRex may grant Sepracor a
sublicense under the Licensed Patents pursuant to Section 3.3 of the Jacobsen
License Agreement.

          NOW THEREFORE, in consideration of the premises and the mutual
covenants contained herein, the parties hereto agree as follows:

          1.   Assignment.  Sepracor hereby assigns its rights and obligations
               ----------                                                     
under the Jacobsen License Agreement to ChiRex, subject to Sepracor's rights to
practice the Licensed Patents pursuant to its sublicense per Section 3 of this
Assignment.   Licensor hereby consents to the assignment of Sepracor's rights
and obligations to ChiRex, and ChiRex agrees to abide by the terms of the
Jacobsen License Agreement and assume all of Sepracor's obligations under said
agreement as of the Effective Date. 

                                       1
<PAGE>
 
However, the foregoing shall not effect, or is not intended to effect, a
novation.

          2.   Notices.  Section 9.2 of the Jacobsen License Agreement shall be
               -------                                                         
amended so that notices to Licensee shall be sent to:

               ChiRex America Inc.
               300 Atlantic Street, Suite 402
               Stamford, CT 06901
               Attention: General Counsel
               Telephone: (203) 351-2300
               Fax: (203) 425-9996

          3.   Sublicense.
               ---------- 

          (a) ChiRex hereby grants Sepracor a worldwide, exclusive (even as to
          ---                                                                 
              ChiRex) sublicense under the Licensed Patents, to develop, make,
              use and sell (i) those compounds included in Exhibit C-1 attached
              hereto and made a part hereof; (ii) Combinatorial Chemistry
              Libraries (as defined below) of chiral or achiral compounds; and
              (c) compounds in the Combinatorial Chemistry Libraries in
              quantities of less than one (1) kilogram. In addition, ChiRex
              further grants to Sepracor a worldwide non-exclusive sublicense
              under the Licensed Patents to manufacture pharmaceutical fine
              chemical intermediates and pharmaceutical active ingredients for
              the clinical and laboratory use of Sepracor and its licensees.
              Such sublicenses shall bear other terms and conditions as set
              forth in the Jacobsen License Agreement and Sepracor hereby
              represents and warrants that it has a copy of the Jacobsen License
              Agreement, is familiar with its terms and will comply with such
              terms. Notwithstanding Section 6.5 of the Jacobsen License
              Agreement, RCT will receive the same royalties on Net Sales Value
              of Licensed Products sold by Sepracor as it would receive on Net
              Sales Value of Licensed Products sold by ChiRex.

          (b) The sublicense granted per subsection (a) above shall include the
          ---                                                                  
              right to sublicense (within the scope of the rights granted
              therein) third parties without ChiRex's consent provided (i) all
              sublicenses granted by Sepracor will be in writing, charge
              sublicensees royalties per Section 6.5 of the Jacobsen License
              Agreement, and expressly require all sublicensees to comply with
              the terms of the Jacobsen License Agreement; (ii) Sepracor
              provides RCT and ChiRex with notice of and a copy of all written
              sublicenses entered into; and (iii) Sepracor and not ChiRex, shall
              remain responsible for the performance,

                                       2
<PAGE>
 
              collection and remission of royalties by all its sublicensees.

          (c) The sublicense granted to Sepracor under this Assignment shall
          ---                                                               
              permit Sepracor, only after prior consultation with ChiRex, to
              grant Nagase & Company, Ltd. a non-exclusive sublicense under the
              Licensed Patents to manufacture Jacobsen catalyst solely in
              connection with the practice of Process Technology (as such term
              is defined in Sepracor's July 24, 1990 Development Agreement with
              Nagase) to manufacture epoxychromans worldwide with the right to
              sell such epoxychromans only in Japan; provided such sublicense
              complies with the requirements of subsection (b) above.

          (d) Sepracor shall have the right to notify ChiRex in writing, at any
          ---                                                                  
              time after Sepracor or one of its licensees begins clinical
              trials, that it wishes to add to Exhibit C-1 any (i) active
              metabolite compound; or (ii) single isomer pharmaceutical
              compound; or (iii) other chiral compound which is identified as
              being from a Combinatorial Chemistry Library; such compounds shall
              be added to Exhibit C-1 subject to the procedures and terms set
              out in Exhibit C of the Technology Agreement.

          (e) As used herein, "Combinatorial Chemistry Library" shall mean any
          ---                                                                 
              group of 25 or more compounds related in structure and synthesized
              contemporaneously from a common intermediate in quantities of no
              more than one hundred grams per compound.

          (f) Licensor hereby consents to the grant of the sublicense to
          ---                                                           
              Sepracor specified herein.

          4.   ChiRex Termination.  Should ChiRex abandon, breach or otherwise
               ------------------                                             
fail to maintain the Jacobsen License Agreement, provided Sepracor is not in
breach of the terms of its sublicense, RCT agrees to grant Sepracor a direct
license on the same terms and conditions set forth in Section 3 so that
Sepracor's use of the Licensed Patents is uninterrupted.

          5.   No Other Changes.  Except as is expressly stated in this
               ----------------                                        
Assignment, the terms of the Jacobsen License Agreement remain in full force and
effect.

          6.   Definitions.  Unless defined specifically herein, all capitalized
               -----------                                                      
terms used in this Assignment shall have the meaning ascribed to such terms in
the Jacobsen License Agreement.

                                       3
<PAGE>
 
          7.   Counterparts.  This Assignment may be executed in one or more
               ------------                                                 
counterparts, each of which shall be deemed an original, but all of which
together will constitute one and the same instrument.

                                 * * * * * * *

                                       4
<PAGE>
 
          IN WITNESS WHEREOF, the parties have caused their duly authorized
representative to execute this Agreement as of the Effective Date.

RESEARCH CORPORATION TECHNOLOGIES, INC.

BY:_______________________
NAME:____________________
TITLE:_____________________



SEPRACOR INC.                            CHIREX AMERICA INC.

BY:_______________________               BY:______________________
NAME:____________________                NAME:___________________
TITLE:_____________________              TITLE:____________________

                                       5
<PAGE>
 
                                  Exhibit C-1
                                  -----------

Terfenadine carboxylate (racemate and single isomers)
R-Ketoprofen
S-Ketoprofen (for use in dentifrice or mouthwash formulations)
R-Albuterol
R,R,-Formoterol
R-Fluoxetine
S-Fluoxetine
S-Oxybutynin
R-Onybutynin
S-Doxazosin
Norastemizole
Norcisapride (racemate and single isomers)
R-Ondansetron
S-Ondansetron
(-)-Amlodipine
Pantoprazole single isomers
Ketoconazole single isomers
Itraconazole single isomers
Descarboethoxyloratadine
Lomefloxacin single isomers
Ketorolac single isomers
Etodolac single isomers
Metoprolol single isomers
Cisapride (racemate and single isomers)
Salmeterol single isomers
Zoplicone single isomers
Sibutramine single isomers
Cetirizine single isomers
Zileuton single isomers
Hydroxyitraconazole single isomers (other than racemic
     hydroxyitraconazole)
Glycopyrrolate single isomers
Clidinium single isomers
Tridihexethyl single isomers
Trihexplenidyl single isomers
Desformoterol single isomers
Desethyloxybutynin single isomers
Procyclidine single isomers
Lansoprazole single isomers
Bupropion single isomers
Rabeprazole single isomers
Hydroxyomeprazole single isomers
Omeprazole single isomers

                                       6
<PAGE>
 
                                                                    EHIBIT 10.43

                                                                  Execution Copy

                                   ASSIGNMENT
                                   ----------


          THIS ASSIGNMENT has an effective date of May 19, 1998 (the "Effective
Date") and is made between MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 77
Massachusetts Avenue, Cambridge, MA 02139 ("Licensor"), SEPRACOR INC., 33 Locke
Drive, Marlborough, MA 01752 ("Sepracor") and CHIREX AMERICA INC., 300 Atlantic
Street, Suite 402, Stamford, Connecticut 06901 ("ChiRex").

          WHEREAS, Licensor and Sepracor are parties to a License Agreement,
dated June 21, 1991 as amended on June 21, 1991 and by a Letter of Understanding
dated May 20, 1996 (collectively referred to herein as the "License Agreement")
covering MIT Case Number 4253, "Optically Active Derivative of Glycidol" by K.
Barry Sharpless and Tetsuo H. Onami; and MIT Case Number 4310, "Optically Active
Derivatives of Glycidol", by K. Barry Sharpless, Janice Klunder and Tetsuo H.
Onami; and

          WHEREAS, ChiRex has practiced the Patent Rights under the License
Agreement as a sublicensee of Sepracor under a Technology Transfer and License
Agreement between Sepracor and SepraChem (a predecessor of ChiRex) effective
January 1, 1995 (the "Technology Agreement") and the aforementioned Letter of
Understanding; and

          WHEREAS, since the merger of Sepracor's Seprachem subsidiary into
ChiRex, ChiRex has exploited the Patent Rights to a greater extent than Sepracor
and the parties to this Amendment Agreement have agreed that Sepracor shall
assign the License Agreement to ChiRex, and ChiRex shall, from the Effective
Date, become the Licensee under such agreement pursuant to the terms set forth
herein; and

          WHEREAS, Licensor has also agreed that ChiRex may grant Sepracor a
sublicense under the Patent Rights pursuant to Section 2.3 of the License
Agreement.

          NOW THEREFORE, in consideration of the premises and the mutual
covenants contained herein, the parties hereto agree as follows:

          1.  Assignment.  Sepracor hereby assigns its rights and obligations
              ----------                                                     
under the License Agreement to ChiRex, subject to Sepracor's 


                                       1
<PAGE>
 
                                                                  Execution Copy


rights to practice the Patent Rights pursuant to its sublicense per Section 3 of
this Assignment. Licensor hereby consents to the assignment of Sepracor's rights
and obligations to ChiRex, and ChiRex agrees to abide by the terms of the
License Agreement and assume all of Sepracor's obligations under said agreement
as of the Effective Date. The term Licensee as defined in the License Agreement
shall hereinafter mean ChiRex.

          2.  Notices.  Article XIV of the License Agreement shall be amended so
              -------                                                           
that notices to Licensee shall be sent to:

               ChiRex America Inc.
               300 Atlantic Street, Suite 402
               Stamford, CT 06901
               Attention: General Counsel
               Telephone: (203) 351-2300
               Fax: (203) 425-9996

          3.   Sublicense.
               ---------- 

          (a)  ChiRex hereby grants Sepracor a worldwide, exclusive (even as to
          ---                                                                 
               ChiRex) sublicense under the Patent Rights, to develop, make, use
               and sell (i) those compounds included in Exhibit C-1 attached
               hereto and made a part hereof; (ii) Combinatorial Chemistry
               Libraries (as defined below) of chiral or achiral compounds; and
               (c) compounds in the Combinatorial Chemistry Libraries in
               quantities of less than one (1) kilogram. In addition, ChiRex
               further grants to Sepracor a worldwide non-exclusive sublicense
               under the Patent Rights to manufacture pharmaceutical fine
               chemical intermediates and pharmaceutical active ingredients for
               the clinical and laboratory use of Sepracor and its licensees.
               Such sublicenses shall bear the terms and conditions as set forth
               in Articles II, V. VII, VIII, IX, X, XII and XV of the and
               Sepracor hereby represents and warrants that it has a copy of the
               License Agreement, is familiar with its terms and will comply
               with such terms. Notwithstanding Section 4.1(e) of the License
               Agreement, Licensor will receive the same royalties on Net Sales
               of Licensed Products by Sepracor as it would receive on Net Sales
               on Licensed Products by ChiRex.


                                       2
<PAGE>
 
                                                                  Execution Copy


          (b)  The sublicense granted per subsection (a) above shall include the
          ---                                                                  
               right to sublicense (within the scope of the rights granted
               therein) third parties without ChiRex's consent provided (i) all
               sublicenses granted by Sepracor will be in writing, charge
               sublicensees royalties per Section 4.1(e) of the License
               Agreement, and expressly require all sublicensees to comply with
               the terms of the License Agreement; (ii) Sepracor provides
               Licensor and ChiRex with notice of and a copy of all written
               sublicenses entered into; and (iii) Sepracor and not ChiRex,
               shall remain responsible for the performance, collection and
               remission of royalties by all its sublicensees.

          (c)  Sepracor shall have the right to notify ChiRex in writing, at any
          ---                                                                   
               time after Sepracor or one of its sublicensees begins clinical
               trials, that it wishes to add to Exhibit C-1 any (i) active
               metabolite compound; or (ii) single isomer pharmaceutical
               compound; or (iii) other chiral compound which is identified as
               being from a Combinatorial Chemistry Library; such compounds
               shall be added to Exhibit C-1 subject to the procedures and terms
               set out in Exhibit C of the Technology Agreement.

          (d)  As used herein, "Combinatorial Chemistry Library" shall mean any
          ---                                                                 
               group of 25 or more compounds related in structure and
               synthesized contemporaneously from a common intermediate in
               quantities of no more than one hundred grams per compound.

          (e)  Licensor hereby consents to the grant of the sublicense to
          ---                                                           
               Sepracor specified herein.

          4.   ChiRex Termination.  Should ChiRex abandon, breach or otherwise
               ------------------                                             
fail to maintain the License Agreement, provided Sepracor is not in breach of
the terms of its sublicense, Licensor agrees to grant Sepracor a direct license
on the same terms and conditions set forth in Section 3 so that Sepracor's use
of the Patent Rights is uninterrupted.

          5.   No Other Changes.  Except as is expressly stated in this
               ----------------                                        
Assignment, the terms of the License Agreement remain in full force and effect.


                                       3
<PAGE>
 
                                                                  Execution Copy


          6.   Definitions.  Unless defined specifically herein, all capitalized
               -----------                                                      
terms used in this Assignment shall have the meaning ascribed to such terms in
the License Agreement.

          7.   Nagase Sublicense.  The non-exclusive sublicense granted by
               -----------------                                          
Sepracor to Nagase & Company, Ltd. Under the Patent Rights per the Amendment
Agreement dated June 5, 1997 between such parties shall continue in full force
and effect; provided however that Sepracor, and not ChiRex, shall be responsible
for ensuring Nagase's compliance with such sublicense and for collecting and
remitting royalties from Nagase to MIT.

          8.   Counterparts.  This Assignment may be executed in one or more
               ------------                                                 
counterparts, each of which shall be deemed an original, but all of which
together will constitute one and the same instrument.

                                 * * * * * * *


                                       4
<PAGE>
 
                                                                  Execution Copy

          IN WITNESS WHEREOF, the parties have caused their duly authorized
representative to execute this Agreement as of the Effective Date.

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

By:______________________
Name:____________________
Title:___________________



Sepracor Inc.                            ChiRex America Inc.

By:_______________________               By:_______________________
Name:_____________________               Name:_____________________
Title:____________________               Title:____________________

                                       5
<PAGE>
 
                                                                  Execution Copy


                                  Exhibit C-1
                                  -----------

Terfenadine carboxylate (racemate and single isomers)
R-Ketoprofen
S-Ketoprofen (for use in dentifrice or mouthwash formulations)
R-Albuterol
R,R,-Formoterol
R-Fluoxetine
S-Fluoxetine
S-Oxybutynin
R-Onybutynin
S-Doxazosin
Norastemizole
Norcisapride (racemate and single isomers)
R-Ondansetron
S-Ondansetron
(-)-Amlodipine
Pantoprazole single isomers
Ketoconazole single isomers
Itraconazole single isomers
Descarboethoxyloratadine
Lomefloxacin single isomers
Ketorolac single isomers
Etodolac single isomers
Metoprolol single isomers
Cisapride (racemate and single isomers)
Salmeterol single isomers
Zoplicone single isomers
Sibutramine single isomers
Cetirizine single isomers
Zileuton single isomers
Hydroxyitraconazole single isomers (other than racemic hydroxyitraconazole)
Glycopyrrolate single isomers
Clidinium single isomers
Tridihexethyl single isomers
Trihexplenidyl single isomers
Desformoterol single isomers
Desethyloxybutynin single isomers
Procyclidine single isomers
Lansoprazole single isomers

                                       6
<PAGE>
 
                                                                  Execution Copy

Bupropion single isomers
Rabeprazole single isomers
Hydroxyomeprazole single isomers
Omeprazole single isomers


                                       7