Prostaglandin I.sub.2 (PGI.sub.2, prostacyclene) is a compound discovered 
by J. R. Vane et al in 1976, biosynthesized from arachidoric acid via endo 
peroxide (PGH.sub.2 or PGG.sub.2) in the arterial wall, attracting 
attention as a substance having a strong platelet aggregation inhibiting 
activity and vasodilating activity. 
(Refer to C & EN, Dec. 20, 1976, p 17 and S. Moncada, R. Gryglewski, S. 
Bunting, J. R. Vane, "Nature," 263,633 (1976)), 
##STR1## 
Because of the unstable exoenol ether structure, PGI.sub.2 is very unstable 
even in a neutral aqueous solution, changing into 6-oxo PGF.sub.1.alpha. 
having hardly any physiological activity. The instability of PGI.sub.2 
becomes a serious drawback when it is used as a medicinal compound. 
Further, PGI.sub.2 is unstable inside an organism, having another drawback 
in that its physiological action has no continuity. 
It is an object of this invention to overcome these disadvantages of 
PGI.sub.2. 
SUMMARY OF THE INVENTION 
The above-mentioned object of the invention is achieved by a compound of 
the formula 
##STR2## 
wherein R.sub.1 denotes a carboxyl group or a functional derivative 
thereof, 
##STR3## 
A denotes (i) --(CH.sub.2).sub.n --, preferably --CH.sub.2 CH.sub.2 --, 
(ii) --CH.dbd.CH--CH.sub.2 --, 
(iii) --CH.sub.2 CH.dbd.CH-- or 
(iv) --CH.sub.2 --O--CH.sub.2 --, wherein n is an integer of 1-3, 
Y denotes hydrogen, alkyl having 1-4 carbon atoms, chlorine, fluorine, 
bromine, formyl, methoxy or nitro, 
B denotes to all A and Y 
(i) 
##STR4## 
(ii) 
##STR5## 
further, when A is (ii) --CH.dbd.CH--CH.sub.2 --, (iii) --CH.sub.2 
CH.dbd.CH--, (iv) --CH.sub.2 --O--CH.sub.2 -- or when Y is an alkyl group 
having 1-4 carbon atoms, chlorine, fluorine, bromine, formyl, methoxy or 
nitro, B further denotes 
(iii) 
(i) 
##STR6## 
wherein R.sub.9 denotes hydrogen or an alkyl group having 1-4 carbon 
atoms, 
R.sub.8 denotes hydrogen, acyl having 1-12 carbon atoms, aroyl having 6-15 
carbon atoms, tetrahydropyranyl, tetrahydrofuranyl, 1-ethoxy ethyl or 
t-butyl, 
X denotes 
(i) --CH.sub.2 CH.sub.2 --, 
(ii) --CH.dbd.CH-- (trans) or 
(iii) --C.tbd.C-- 
R.sub.10 denotes 
(i) straight chain alkyl having 4-10 carbon atoms, or 
(ii) 
##STR7## 
wherein Z denotes valence bond, or straight chain or branched alkylene 
which may be represented by C.sub.t H.sub.2t, wherein t denotes an integer 
of 1-5, further, m denotes an integer of 5-12, R.sub.3 denotes hydrogen or 
alkyl having 1-5 carbon atoms, and n' denotes an integer of 1-3, or 
(iii) --Z--Ar.sub.2, wherein Z is the same as defined above, and Ar.sub.2 
denotes phenyl, .alpha.-naphthyl, .beta.-naphthyl or at least one 
chlorine, bromine, fluorine, trifluoromethyl, alkyl having 1-4 carbon 
atoms, nitro, methoxy, phenyl or phenoxy-substituted phenyl, 
R.sub.13 denotes 
(i) branched alkyl having 5-10 carbon atoms, or 
(ii) --C.sub.t H.sub.2t OR.sub.14, wherein C.sub.t H.sub.2t is the same as 
defined above, and R.sub.14 denotes straight chain or branched alkyl 
having 1-5 carbon atoms, 
##STR8## 
or --Z--Ar.sub.2, wherein Z, m, R.sub.3, n' and Ar.sub.2 are the same as 
defined above, or 
(iii) 
##STR9## 
(wherein C.sub.t H.sub.2t is the same as defined above, R.sub.15 and 
R.sub.16 denote hydrogen, methyl, ethyl, propyl or butyl group, 
W denotes 
(i) --CH.sub.2 CH.sub.2 -- or 
(ii) --CH.dbd.CH-- (trans) 
and the general formula denotes d form, l form or dl form.)