Stereoselective Synthesis of an Analogue of Podophyllotoxin by an Intramolecular Diels-Alder Reaction

Article abstract of DOI:10.1021/jo00207a034

The dihydrobenzocyclobutene 90 having a 4-hydroxycrotonate unit attached via an ester linkage as an internal dienophile can be cyclized to a 3:1 mixture of the trans lactone 92 (an analogue of podophyllotoxin, 1) and the cis lactone 93.This stereoselective reaction proceeds via the intermediacy of the o-quinodimethane 91 which cyclizes from the endo transition state 91n in preference to the exo-one 91x, presumably because of stabilization of the former by secondary orbital overlap.This result provides evidence that a proposed general route to the synthesis of podophyllotoxin, 1, and its analogues via the internal cycloaddition of the o-quinodimethane 8 to 9n may prove successful.Several possible approaches to the synthesis of the trans-2-aryldihydrobenzocyclobutenol 4 are described.The benzyne 11 was prepared and underwent <2 + 4> but no <2 + 2> cycloadditions.Although the 2-bromobenzocyclobutenone 23 could be synthesized in an efficient manner, it proved impossible to convert it into 4 by means of the aryl organometallic reagents 22ab.The bromo epoxide 52 was prepared and subjected to metal-halogen exchange and Lewis acid catalyzed epoxide rearrangement in an attempt to prepare 4.The aldehyde 56 was obtained in this reaction, clearly indicating that the desired intermediate 54 had been formed but could not be trapped under these conditions.Two ring contraction routes to 4 are also described, both beginning with the 1-indanone 74 prepared in good yield from piperonal 14.The diol monomesylate 78, prepared from 74, suffered base-catalyzed E2 elimination rather than the desired rearrangement to 80.The diazo ketone 83 underwent Wolff rearrangement to give the desired ester 84, but only in 7percent yield.Two interesting transformations were observed in these ring contraction schemes, namely the formation of the oxathiole dioxide 77 on mesylation of the ketol 75 and the preparation of the diazirene 86 on photolysis of the diazo ketone 83 at long wavelengths.The ester 84 was then saponified to the acid 87 which was coupled with methyl 4-hydroxycrotonate, 5, to give 90.The assignment of the structures of the products of thermolysis of 90 was based on high field 1H NMR and analogy to the spectra of similar compounds in the literature.