Recent reports by Liottala and Smithlb have described the SN2-type opening of lactones with various forms of the sodium salt of the anion derived from phenyl selenol. In connection with other work, we were interested in converting the lactone e to diene 2 and decided to investigate the use of this methodology.
(E)-Ethylidene-y-lactone (lb)2 (13.5 mmol) was reacted with 1.1 equivalents of potassium phenyl selenoate (PhSeK) in 10% HMPA/THF at 80'1~ for 6 hours. The selenoate was prepared by heating a dry THF solution (15 ml) of diphenyldiselenide (7.7 mmol) in the presence of potassium metal (14.9 mg atom) at 80°C for 3 hours in a resealable sealed tube.
3 Progress of the reaction was monitored by observing the disappearance of lactone and appearance of carboxylate stretching frequencies in the ir spectra of crude aliquots. Isolation of the crude acid (see procedure in ref. la), esterification (CH2N2, Et20), and purification by chromatography on silica gel (6:l/hexanes:EtOAc) provided the selenide 3b4 in 71% yield. Interestingly, a slight amount (G%) of isomerization of fi to (Z)-ethylidene-y-lactone (2) could be detected by gas chromatographic analysis of the reaction mixture at low conversion. This implies the reversible formation of a lactone enolate ion from Michael attack of PhSeK to the unsaturated lactone However, no products resulting from net Michael addition to & nor from SN2 opening of the (Z)lactone 2 were ever detected. The only undesirable material isolated from this reaction was the diselenide Q4 (ZJide infra).
Nucleophilic opening of the lactone ring in the more complex substrate ,@' provided the phenylselenide k4 (62%) after esterification and chromatography. Again, no Michael adducts or E-Z isomerized products were observed. The presence of the free hydroxyl group in & also did not interfere with the reaction, nor was any diselenide product analogous to &i observed.
We initially used the potassium rather than sodium salt of phenyl selenol almost exclusive