STEREOSELECTIVE INTRAMOLECULAR CYCLIZATION OF BETA-ALKOXYCARBONYL-OMEGA-FORMYLALLYSILANES INTO BICYCLIC ALPHA-METHYLENE-GAMMA-LACTONES

Alpha-Methylene-gamma-lactones fused to five- and six-membered carbocy cles were efficiently synthesized from beta-ethoxycarbonyl-omega-formy lallylsilane derivatives by means of the intramolecular Hosomi reactio n. The formylated allylsilanes (11a, b, 12a and b) were synthesized fr om ethyl beta-trimethylsilylpropionate and omega-tetrahydropyranyloxyp entanal (4a) and -hexanal (4b) in several steps. The cyclization react ion of these aldehydes was performed under mild conditions with a high degree of stereocontrol. Treatment of the (E)- and (Z)-isomers of the allylsilanes (11a and 12a) with titanium tetrachloride or boron trifl uoride etherate gave a five-membered cis-hydroxy ester (13a). Treatmen t of the (Z)-allylsilane derivative (11b) with titanium tetrachloride afforded a six-membered cis-hydroxy ester (13b) as a major product tog ether with the trans-isomer (14b), and treatment with boron trifluorid e etherate selectively gave the cis-isomer (13b). On the other hand, t reatment of (E)-allylsilane (12b) with titanium tetrachloride selectiv ely gave the cis-hydroxy ester (13b), while the use of boron trifluori de etherate exclusively afforded the trans-isomer (14b). These stereos electivities can be explained in terms of chelated transition models. Lactonization of these hydroxy esters afforded the corresponding fused alpha-methylene-gamma-lactones (16a, 17a and b) in good yields.