3aH-indenes. Part 5. Preparation and reactions of 3-methoxy- and 3-trimethylsiloxy-3a-substituted-3aH-indenes

Abstract

Previous work on the synthesis and cycloaddition of 3a-methyl-3aH-indenes (1a) and (1b) has been extended to substituents other than methyl at the ring junction. However, the increased migratory aptitudes of these substituents limits the scope of the 3aH-indene chemistry; thus rearrangement of 3aH-indenes (1) to their 1H-isomers (2) is sometimes so rapid that the 3aH-indenes could not be isolated or intercepted in cycloaddition reactions. Dissolving metal reduction and alkylation of indanone gave the dienones (3) where R = Et, Pr¹, PhCH₂, CH₂CHCH₂, HCCCH₂, and MeO₂CCH₂, and these were converted into the trienones (9)viaα-phenylselenation. The trienone (9a) was converted into the 3aH-indene (1c) which could be intercepted as the [8 + 2] cycloadduct (10). The 3aH-indenes (1e) and (1h) were similarly generated from the trienones (9c) and (9g), but rearranged too rapidly to the 1H-isomers (2) to be intercepted. The prop-2-ynyl derivative (9e) gave the 1H-isomer (11) directly on enolisation, and the allyl derivative (9d) also rearranged rapidly to give the indanone (12). Silylation of the isopropyl trienone (9b) with chlorotrimethylsilane generated the 3aH-indene (1d), but this underwent a rapid [1,5] shift of isopropyl at room temperature; with trimethylsilyl trifluoromethane-sulphonate as the silylating agent at –23 °C, the 3aH-indene could be intercepted with PTAD to give the adduct (15) and with 2-chloroacryloyl chloride to give, after standard transformations, the tricyclic ketone (17). Similarly the methoxycarbonylmethyl (9g) and the hydroxyethyl (22) substituted trienones were converted into the tricyclic adducts (18), and (24) and (25), respectively.