ENOLIC ORTHO ESTERS - VIII - SYNTHESIS OF BIS(METHOXYMETHYL)-1,4,6-TRIOXASPIRO[4.5]DEC-7-ENE

Enolic ortho ester (2S,3S)-2,3-bis(methoxymethyl)-1,4,6-trioxaspiro [4 .5] dec-7-ene (10b) was synthesized in 65% yield by cycloadditon of ac rolein with the ketene acetal -4,5-bis(methoxymethyl)-2-methylidene-1, 3-dioxolan (7), derived by potassium t-butoxide treatment of (4S,5S)-4 ,5-bis(methoxymethyl)-1,3-dioxolan (4). Lewis acid catalysed reaction (TiCl4, CH2Cl2, -78 degrees) of the enolic ortho ester (10b) with the ketene silyl acetal -phenylmethyl-2-trimethylsilyloxy-4,5-dihydrofuran (15) afforded the formyl keto ester 3-[4',5'-bis(methoxymethyl)-2'-(4 n-2'-yl]-3-phenylmethyl-4,5-dihydrofuran-2(3H)-one (16) (39%) with on ly poor diastereoselectivity. Attempts to make the bis(phenylmethyl) a nalogue (10a) of enolic ortho ester (10b) were thwarted by the finding that the required ketene acetal (9), analogous to (7), could not be g enerated by elimination from (4S,5S)-2-bromomethyl-4, 5-bis (phenylmet hyl)-1,3-dioxolan (6), or from the derived phenyl selenoxide (8), unde r a variety of conditions.