Synthesis of precursors of C4 and C5 cumulenones

Abstract

Bicyclo[2.2.l]hept-5-en-2-one was condensed with 2,2-dimethyl-1,3-dioxan-4,6-dione in the presence of titanium tetrachloride/pyridine to yield 5-(bicyclo[2.2.1]hept-5'-en-2'-ylidene)-2,2-dimethyl-1,3- dioxan-4,6-dione (6a) which on pyrolysis at 520-560º/0.01 mm gave acetone, carbon dioxide, cyclopentadiene and butatrienone. In the same way 5-(7'-oxabicyclo[2.2.1]hept-5'-en-2'-ylidene)-2,2- dimethyl-1,3-dioxan-4,6-dione (6b) was prepared from 7-oxabicyclo[2.2.1]hept-5-en-2-one and on pyrolysis at 570º/0.01 mm it gave acetone, carbon dioxide, furan and butatrienone. Alkylation of 2-trimethylsiloxybicyclo[2.2.1]hepta-2,5 with phenylthiomethyl chloride with titanium tetrachloride as catalyst followed by condensation with 2,2-dimethyl-1,3-dioxan-4,6-dione gave an endo/exo mixture of 5-(3'-phenylthiomethylbicyclo[2.2.1]hept-5'-en-2'-ylidene)-2,2-dimethy 1-1,3-dioxan- 4,6-dione. The structure of the endo isomer was established by X-ray crystallography. Oxidation of the phenylthiomethyl compound with m-chloroperbenzoic acid yielded the sulfoxide which was converted by boiling in carbon tetrachloride into 5-(3'-methylenebicyclo[2.2.l]hept-5'-en-2'-ylidene)- 2,2-dimethyl-1,3-dioxan-4,6-dione (8). Preliminary studies on the fragmentation of compound (8) have shown that at 450º/0.01 mm it is converted into acetone, carbon dioxide and products showing ketene absorption at 2140 cm^-1. The formation of pentatetraenone has not been established with certainty. The 3'-phenylsulfonylmethyl, phenylselenyl and methyl derivatives of (6a) are also described.