Document:

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                                                                 Exhibit 10.23.1

                          LOAN MODIFICATION AGREEMENT

     This Loan Modification agreement is entered into as of December 15, 1999,
by and between Synplicity, Inc. ("Borrower") and Silicon Valley Bank ("Bank").

1.   DESCRIPTION OF EXISTING INDEBTEDNESS: Among other indebtedness which may be
owing by Borrower to Bank, Borrower is indebted to bank pursuant to, among other
documents, an Amended and Restated Loan and Security Agreement, dated September
9, 1998 as may be amended from time to time, (the "Loan Agreement"). The Loan
Agreement provided for, among other things, a Committed Revolving Line in the
original principal amount of Three Million Dollars ($3,000,000), a Committed
Revolving Line in the original principal amount of Two Million Dollars
($2,000,000) and a Term Loan in the principal amount of Three Hundred Forty Four
Thousand Four Hundred Forty Four and 40/100 Dollars ($344,444.40). Defined terms
used by not otherwise defined herein shall have the same meanings as in the Loan
Agreement.

Hereinafter, all indebtedness owing by Borrower to Bank shall be referred to as
the "indebtedness."

2.   DESCRIPTION OF COLLATERAL AND GUARANTIES. Repayment of the Indebtedness is
secured by the Collateral as described in the Loan Agreement.

Hereinafter, the above-described security documents and guaranties, together
with all other documents securing repayment of the indebtedness shall be
referred to as the "Security Documents". Hereinafter, the Security Documents,
together with all other documents evidencing or securing the indebtedness shall
be referred to as the "Existing Loan Documents."

3.   DESCRIPTION OF CHANGE IN TERMS.

     A.  Modification(s) to Loan Agreement

         1.  The profitability covenant as described in Section 6.7 entitled
             "Financial Covenants" is hereby amended to read as follows:

             Profitability. Borrower will be profitable each quarter, except
             that Borrower may suffer losses, provided such losses do not
             exceed:

<TABLE>
<CAPTION>

        Amount                        Quarter Ending
        ------                        --------------
        <S>                           <C>
        $1,800,000                    December 31, 1999
        $1,950,000                    March 31, 2000
        $1,600,000                    June 30, 2000
</TABLE>

     B.  Waiver of Covenant Default.

         1.   Bank hereby waives Borrower's default under the Loan Agreement by
              virtue of Borrower's failure to comply with the Profitability
              covenant as of the quarter ended September 30, 1999. Bank's waiver
              of Borrower's compliance of this covenant shall apply only the
              foregoing period. Accordingly, for the quarter ending December 31,
              1999, Borrower shall be in compliance with this covenant, as
              amended herein.

              Bank's agreement to waive the above-described default (1) in no
              way shall be deemed an agreement by the Bank to waive Borrower's
              compliance with the above-described covenant as of all other dates
              and (2) shall not limit or impair the Bank's right to demand
              strict performance of this covenant as of all other dates and (3)
              shall not limit or impair the Bank's right to demand strict
              performance of all other covenants as of any date.
<PAGE>

4.   CONSISTENT CHANGES. The Existing Loan Documents are hereby amended wherever
necessary to reflect the changes described above.

5.   PAYMENT OF LOAN FEE. Borrower shall pay to Bank a fee in the amount of One
Thousand Dollars ($1,000) (the "Loan Fee") plus all out-of-pocket expenses.

6.   NO DEFENSES OF BORROWER. Borrower (and each guarantor and pledgor signing
below) agrees that, as of the date hereof, it has no defenses against the
obligations to pay any amounts under the indebtedness.

7.   CONTINUING VALIDITY. Borrower (and each guarantor and pledgor signing
below) understands and agrees that in modifying the existing indebtedness, Bank
is relying upon Borrower's representations, warranties, and agreements, as set
forth in the Existing Loan Documents. Except as expressly modified pursuant to
this Loan Modification Agreement, the terms of the Existing Loan Documents
remain unchanged and in full force and effect. Bank's agreement to modifications
to the existing indebtedness pursuant to this Loan Modification Agreement in no
way shall obligate Bank to make any future modifications to the Indebtedness. It
is the intention of Bank and Borrower to retain as liable parties all makers and
endorsers of Existing Loan Documents, unless the party is expressly released by
Bank in writing. No maker, endorser, or guarantor will be released by virtue of
this Loan Modification Agreement. The terms of this paragraph apply not only to
this Loan Modification Agreement, but also to all subsequent loan modification
agreements.

8.   CONDITIONS. The effectiveness of this Loan Modification Agreement is
conditioned upon Borrower's payment of the Loan Fee.

     This Loan Modification Agreement is executed as of the date first written
above.

BORROWER:                                       BANK:

SYNPLICITY, INC.                                SILICON VALLEY BANK

By: /s/ Douglas S. Miller                       By: /s/ Kevin J. Conway
   ----------------------                          --------------------

Name: Douglas S. Miller                         Name: Kevin J. Conway
      -----------------                               ---------------

Title: VP Finance & CFO                         Title: Vice President
       ----------------                                --------------<PAGE>

                                                                    Exhibit 10.2

August 9, 2000

American Home Products Corporation
Five Giralda Farms
Madison, NJ 07940

Attention:  Kenneth J. Martin
            Senior Vice President and
            Chief Financial Officer

                          MEMORANDUM OF UNDERSTANDING
                          ---------------------------
                     FOR HELIX PROJECT FINANCING GUARANTY
                     ------------------------------------

Dear Mr. Martin:

          Immunex Corporation ("Immunex") plans to construct a new corporate
headquarters and research center in Seattle, Washington, known as the "Helix
Project," and to finance such construction through the use of an off-balance
sheet synthetic lease or alternate financing acceptable to AHP (the
"Financing"). The cost of the Financing would be reduced if Immunex's financial
obligations under the Financing, to the financial institution or institutions
providing funds to Immunex (the "Lenders"), were guaranteed by American Home
Products Corporation ("AHP"). In order to achieve such a cost reduction and for
other valuable consideration, you have indicated AHP's willingness to provide
such a guaranty for Immunex's financial obligations under the Financing on the
terms herein described (the "Guaranty"). The execution of this Memorandum of
Understanding ("MOU") will allow Immunex to inform potential Lenders of AHP's
agreement to provide the Guaranty.

          The parties hereby agree as follows:

     1.   AHP agrees, subject to agreement with the Lenders on the terms of a
customary definitive agreement and satisfaction of the other agreements
contained herein, to provide the Guaranty. The amount of AHP's potential
liability under the Guaranty is set forth in Exhibit A hereto. Although the
precise nature and scope of the Guaranty remain to be negotiated and agreed
among AHP, Immunex and the Lenders, if a synthetic lease is used for the
Financing, Immunex's obligations under the Financing will fall into three phases
(construction period, payment of rents, and Financing termination) and can be
summarized as follows:

          a.   Construction Period.  As the construction agent for the lessor (a
     special purpose entity representing the Lenders), Immunex's major
     obligations will likely include:

               .  hiring of necessary architects and engineers;
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               .  hiring of a general contractor or a group to serve as the
                  construction manager;
               .  leasing the land to the lessor;
               .  completing the Helix Project within the prescribed
                  construction period and according to agreed-upon plans and
                  specifications; and
               .  completing the Helix Project within the budgeted total project
                  amount (hard and soft costs).

          b.   Payment of Rents.  Upon completion of construction of the Helix
     Project (or the end of the construction period), Immunex will begin paying
     rent. Immunex's primary obligations during this phase will likely include:

               .  paying monthly rent based on the selected LIBOR based rate.
                  This rent will be paid directly to the Lenders. The rent will
                  be equal to the monthly interest on the loan that is an
                  implicit part of the Financing; and

               .  paying all other occupancy costs, including insurance, taxes,
                  repairs, maintenance, management fees, etc. A synthetic lease
                  would be a "bond lease," meaning that Immunex will pay all
                  costs, including those that are capital in nature (e.g., roof
                  and structure maintenance and repair). Rent will be due even
                  if the property, or a portion thereof, suffers a catastrophic
                  loss or condemnation; provided, however, that the proceeds of
                  insurance and any condemnation award will be made available to
                  Immunex to repair or restore the property.

          c.   Financing Termination.  Upon termination of the Financing,
     Immunex will likely have three options:

               .  enter into another synthetic lease with the same Lenders or
                  with new lenders;
               .  purchase the property for the outstanding loan balance; or
               .  market and sell the property to a third party.

          Immunex could also terminate the Financing by paying a termination fee
to the lessor. This fee will not exceed 90% of the total Helix Project cost. In
most cases, this will prove an inferior option. If the property is sold, Immunex
retains any net sales proceeds in excess of the sum of the outstanding
loan/equity balance and any unpaid rental and other obligations. If the property
is sold and the net sales proceeds are less than the outstanding loan amount,
Immunex must pay the balance remaining. Under no circumstances, will the
duration of the Guaranty exceed eight years.

     2.   Immunex will collateralize the Financing vis-a-vis the Lenders with
$200 million in cash. If Immunex completes an equity offering (an "Offering") in
accordance with the terms of a letter agreement between the parties dated the
date hereof, it will pledge an additional $100

                                       2
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million in cash to secure the Financing. To the extent that the total Helix
Project costs exceed $750 million, Immunex will pledge an additional amount of
cash to secure the Financing equal to the amount the Helix Project costs exceed
$750 million.

     3.   Immunex shall pay AHP an annual fee equal to 3% (or a fee equal to an
annualized rate of 3%) of the portion of Immunex's outstanding obligations under
the Financing that are not cash collateralized, quarterly in arrears calculated
on the basis of the average daily balance of Immunex's outstanding obligations
not cash collateralized. If Immunex completes an Offering and satisfies its
obligations to cash collateralize the Guaranty with an additional $100 million
of the proceeds of such Offering, then such fee shall be reduced to 1% of that
portion of Immunex's outstanding obligations under the Financing in excess of
$300 million, payable on the same basis as set forth above. If a breach by
Immunex of any of the terms of the Financing or related documentation (including
documents entered into with AHP as described below) remains uncured following
applicable cure periods or AHP makes a payment under the Guaranty, such fee
shall increase by 1.5%; provided, however, that such fee shall be subsequently
reduced by 1.5% if such breach is cured. AHP expects the Financing will contain
customary affirmative and negative covenants and events of default consistent
with and appropriate for a transaction such as the Financing and a borrower of
the size, credit quality and market capitalization of Immunex as determined
through arms-length negotiation. In the event such customary covenants are not
contained in the Financing, Immunex will make such covenants directly to AHP.

     4.   AHP will have a security interest in the Helix Project and cash
pledged by Immunex to collateralize the Financing subordinate only to the
Lenders. AHP will enter into a customary intercreditor agreement with the
Lenders. AHP will have the right to repay all obligations due by Immunex under
the Financing at any time after a breach of Immunex's payment obligations in
respect to an aggregate amount of $10 million or more under the Financing
following customary cure periods; provided, however, that such right shall
terminate with respect to such breach in the event that such breach is cured.
The terms of the Financing shall include that the Lenders shall look to the cash
collateral prior to making a claim on the Guaranty. If the Lenders have been
paid in full under the Financing and AHP has made a payment under the Guaranty,
AHP will have first and unfettered rights to foreclose on the Helix Project and
cash pledged by Immunex to collateralize the Financing to satisfy and be repaid
for any amounts paid under the Guaranty.

     5.   AHP shall use reasonable good faith efforts to enter into a definitive
agreement containing the Guaranty described in Exhibit A as soon as possible
after the Lenders are identified.

     6.   While this MOU is effective, one party (the "Disclosing Party") may
disclose to the other party (the "Receiving Party") confidential or proprietary
information of the Disclosing Party ("Confidential Information"). For Immunex,
Confidential Information may include, but will not necessarily be limited to,
financial information about Immunex, specifications for the Helix Project or
other information related to Immunex's business. The Receiving Party shall hold
the Confidential Information in confidence, and shall not disclose it to any
third party, unless (a) such Confidential Information becomes public through no
fault of the Receiving Party,

                                       3
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(b) the Disclosing Party consents in writing to the disclosure of the
Confidential Information, or (c) the Receiving Party is required by law to
disclose the Confidential Information, provided that the Receiving Party gives
prior written notice thereof to the Disclosing Party, to the extent reasonably
possible. Further, the Receiving Party will not use the Confidential Information
for any purpose other than to perform its obligations hereunder. This
confidentiality provision shall survive termination or expiration of this MOU
for a period of four (4) years.

     7.   This MOU shall be governed by, and construed in accordance with, the
laws of the State of New York.

     8.   This MOU constitutes the entire agreement between the parties with
respect to the subject matter hereof and supersedes all prior agreements and
undertakings, both written and oral, between the parties with respect to the
subject hereof.

     9.   The parties intend that the provisions of this MOU are binding.

     10.  This MOU may be executed and delivered (including by facsimile
transmission) in one or more counterparts, and by AHP and Immunex in separate
counterparts, each of which when executed and delivered shall be deemed to be an
original but all of which taken together shall constitute one and the same
agreement.

          If you are in agreement with the terms of this MOU, please sign both
originals in the space indicated below and return one fully-executed original.

Sincerely,                               Agreed to by:

IMMUNEX CORPORATION                      AMERICAN HOME PRODUCTS CORPORATION

By:  /s/ David A. Mann                   By:  /s/ Kenneth J. Martin
   ---------------------------------        ------------------------------------
   David A. Mann                            Kenneth J. Martin
   Senior Vice President and                Senior Vice President and
   Chief Financial Officer                  Chief Financial Officer

Date:  Aug. 9, 2000                      Date:  August 9, 2000
     -------------------------------          ----------------------------------

                                       4
<PAGE>

                                   EXHIBIT A
                                   ---------

     This exhibit identifies the potential and maximum liabilities American Home
Products Corporation ("AHP") would assume as guarantor of Immunex's financial
(and not operational or other performance) obligations under the proposed
Financing.

     Following a default by Immunex, Immunex would be liable for up to 90% of
the amount financed (approximately $750 million dollars under the currently
proposed facility).  Prior to an Offering, up to $550 million of this amount
would be guaranteed by AHP.  Subsequent to an Offering, $450 million would be
guaranteed by AHP.

     AHP's liability for the Immunex default will be reduced by the $200 million
(or $300 million subsequent to an Offering) of cash collateral which Immunex has
agreed to pledge as security for its obligations. The terms of the Financing
shall include that the Lenders shall look to the cash collateral prior to making
a claim on the Guaranty, such that the 90% "maximum recourse amount" guaranteed
by AHP will be reduced by the Immunex cash collateral.

     Additionally, AHP's liability is mitigated in one of two ways:  (1) upon an
Immunex default, the lessor is obligated to mitigate Immunex's damages by
remarketing the Property for a commercially - reasonable purchase price; (based
upon our Seattle market analysis attached as Schedule 1, we believe that the
market value for the financed improvements will not be less than 85% of the
financed amount, even in a worst-case scenario; the $200 million collateral
substantially exceeds this "worst-case" 15% decrease in market value); or (2)
alternatively, Immunex or AHP could purchase the property out of the synthetic
lease for an amount equal to all amounts due under the synthetic lease documents
plus any break funding amount and then, if they choose, to sell the property to
a third party.  Again, since Immunex is cash collateralizing the first $200
million (or $300 million subsequent to an Offering) of the loan, AHP will only
face exposure if the value of the property has decreased by more than $200
million (or $300 million subsequent to an Offering) at the time of the default,
which we have determined to be a remote possibility.

                                       5

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