In the past five years different approaches I to the synthesis of the prostanoic acid skeleton have emerged from various laboratories. Some of these led to the synthesis of natural 2 prostaglandins .
This communication reports synthesis of some prostanolc acids using a novel approach: the photoaddition between u,B-unsaturated ketones.
It has been reported 3 that mixed photoaddltion of an ~,B-unsaturated ketone and an olefln can compete favourably with the photodimerisation process. However, reports of such mixed photo-4 addition between two different a,~-unsaturated ketones are scarce .
Cyclopentenone ! (R= -(CHa)6COOCH3)5 and chlorovinyl ketone 6 ~ (bp 75-770/0.5 mm) were irradiated (O.5:1.4 moles respectively) with a 500 watt high pressure Hanovia burner, in the presence of a pyrex filter, at 30-35 ° , over a period of 35-40 hr. After the recovery of the unchanged starting materials, (by distillation) followed by chromatographic purification on a silica gel (Merck 0.05-0.2 mm) column yielded photoadduct ~ (35%, R= -(CH2)GCOOCH3, R 1 = CI; Found: CI, 8.85; ~ 4.95 min. with a shoulder at 5.2 min. See also Table I). The assignment of ci..ss stereochemistry at the ring Junctions emanates from analogy with earlier 3 work. The assignment of head to head (syn) orientation of the two ketone functions follows from an analysis of the nmr 7 spectrum. A iOO Mc spectrum demonstrated signals at 4.58 (q, H2, Ji,2 = J2,3 = 7 Hz); 3.666 (s, -OCH3; 3.178 (d, H1, J1,2 = 7 Hz) and 0.888 (poor t, -CH3). The H 2 proton quartet collapsed to a doublet (J=6.5 Hz), upon decoupling at the resonance frequency of the proton H I.
Conversely, on irradiation at the resonance frequency of H2, the doublet of ~ gave rise to a singlet 8.
A further support for our structural assignments is lent by the following transformations.