The benxopyranones 1 and 3 reacted with 2.3-dimethyl-1,3-butadiene in the presence of titanium (IV) chloride to give the corresponding (4 + 2) cycloadducts 8 and 11, the former undergoing facile deformylation to give 9 and 10. Compounds 1.3. and 4 underwent efficient uncatalysed cycloaddition to lmethoxy-3-(nimcthy1sily1oxy>1,3-butadiene 12 to give the respective adducts 13.14, and 18 as mixtures of C-l stereoisomers. Heating the 3-arylsulphinylchrne 5 with the dicne 12 afforded 3-hydroxyxanthone 23 in 50% yield. the presumed cycloaddition -elimination sequence constituting a new route to xanthone systems. Desilylation of 13.14, and 18 in acidic media provided 25.26, and 27 respectively.
Chromones bearing electron withdrawing substituents at C-3 are highly functional molecules, capable of reacting as Michael acceptors12 or hetemdienes,3 and undergoing various rearrangements on treatment with nucleophiles.4 However, their use as 2n components in (4 + 2) cyc1oadditions has received surprisingly little attention5 despite their obvious potential in such a r61e. We have therefore studied the reactions of 2,3-dimethyl-1,3-butadiene and I-methoxy-3-(trime.thylsilyloxy)-1,3butadiene (Danishefsky's diene) with the representative pyranone systems 1 -5. Our results, herein described, confum that 3076 P. J. CREMNS er al.
Initial experiments were carried out using 2,3dimethyl-1,3-butadienc.
On treatment of the aldehyde 1 with a IO-fold excess of the dienc in refluxing dichloromethane, no reaction could be detected after several days, white the inclusion of a catalytic amount of titanium(TV) chloride in the solution resulted in the formation of a complex mixture as judged by thin layer chromatography (t.1.c.). It was suspected that the initial adduct 8, a tertiary bketoaldehyde, would be prone to deformylation under the conditions of the reaction; indeed, increasing the reaction temperature (so as to encourage the deformylation process) produced a cleaner mixture from which the tricycles 9 and 10 (total 41%) were isolated. With the acid 2 similar problems were encountered, the reaction requiring TiQcatalysis and producing a complex mixture of products. However, under the same conditions the ester 3 and 2.3-dimethyl-1,3-butadiene cleanly gave the stable cycloadduct 11(84%).