3aH-indenes. Part 6. Cycloaddition of 3-methoxy- and 3-trimethylsiloxy-3a-methyl-3aH-indenes to heterodienophiles; approaches to aza[10]annulenes

Abstract

3-Methoxy-3a-methyl-3aH-indene (4a) reacts with dimethyl azodicarboxylate and with bistrichloroethyl azodicarboxylate to give the [8 + 2]-cycloadducts (5a) and (5c), respectively. 3-Trimethylsiloxy-3a-methyl-3aH-indene (4b) reacts similarly with the same two azo esters to give cycloadducts (5b) and (5d) which, on isolation, give the ring-opened ketones (6a) and (6b). Reduction of the bistrichloroethyl ester (5c) with zinc followed by oxidation with copper(II) chloride regenerates the 3a-methylindene (4a); the proposed pathway for this is shown in Scheme 2. The 3a-trimethylsiloxyindene (4b) undergoes [8 + 2]-cycloaddition to trichloroacetonitrile to give the adduct (10) which is hydrolysed by acid to the diketone (11). 3a-Methylindene (4a) reacts with toluene-p-sulphonyl isocyanate to give the cycloadduct (12) and with chlorosulphonyl isocyanate to give, after hydrolysis, the [8 + 2]- cycloadducts (13) and (14) which are converted by methyl fluorosulphonate into the cyclic imidates (16) and (17) respectively. Attempts to transform these various [8 + 2]-adducts into mono-aza and di-aza analogues of 7b-methyl-7bH-cyclopent[cd]indene were unsuccessful.