ALBERT

Notes: Synthesis of new polycyclic compounds by means of intramolecular Diels-Alder reactions of cyclohexa-2,4-dien-1-one derivativesThermal rearrangement of mesityl penta-2,4-dienyl ether (1), consisting of the isomers E (93%) and Z (7%), furnished, besides mesitol, the two mesityl penta-1,3-dienyl ethers 2 (24%) and 3 (3%), and the two tricyclic ketones 4 (4,5%) and 5 (12,5%) (Scheme 1). A probable mechanism for this formation of 2 involves a [1,5]-hydrogen shift in (Z)-1. Isomerisation of (E)-1 to (Z)-1 at 145° occurs via reversible sigmatropic [3,3]- and [5,5]-rearrangements of (E)-1 to the cyclohexadienones 38 and 39 respectively (see Chapter A p. 1710, and Scheme 15). Formation of 3 from either (Z)-1 or 2 is rationalized by a series of pericyclic reactions as outlined in Chapter A and Scheme 16.The tricyclic ketones 4 and 5 are undoubtedly formed by internal Diels-Alder reactions of the 6-pentadienyl-cyclohexa-2,4-dien-1-one 6 (Scheme 2). In fact, at 80° 6 is converted into 4 (5%) and 5 (35%). At 80° the cyclohexadienone derivative 7 furnished the corresponding tricyclic ketones 8 (15%) and 9 (44%) (Scheme 2). 5 and 9 contain a homotwistane skeleton. 8 and 9 are easily prepared by reaction of sodium 2,6-dimethylphenolate with 3-methyl-penta-2,4-dienyl bromide at ambient temperature, followed by heating, and finally separation by cristallization and chromatography.The cyclohexadienones 6 and 7 have mainly (E)-configuration. Here too (E) → (Z) isomerization is a prerequisite for the internal Diels-Alder reaction, and this partly takes place intramolecularly through reversible Claisen and Cope rearrangements (Scheme 17). On the other hand, experiments in the presence of 3,5-d2-mesitol have shown (Table 1) that intermolecular reactions, involving radicals and/or ions, are also operating (see Chapter B, p. 1712).Two different modi (I and II) exist for intramolecular Diels-Alder reactions (Scheme 18). Whereas only modus I is observed in the cyclization of 5-alkenyl-cyclohexa-l,3-dienes, in that of (2)-cyclohexadienones 6 and 7 (Scheme 2) both modi are operating. Only in modus 11-type transitions is the butadienyl conjugation of the side chain retained, so that modus 11-type addition is preferred (Chapter C p. 1716).Analogously to the synthesis of the tricyclic ketones 4, 5, 8 and 9, the tricyclic ketone 15 (Scheme 4) and the tetracyclic ketone 11 (Scheme 3) are prepared from mesitol, pentenyl bromide and cycloheptadienyl bromide, respectively.From the polycyclic ketones derivatives such as the alcohols 16, 17, 18, 19, 23, 24 and 25 (Schemes 9 and 11), policyclic ethers 20, 21, 22 and 26 (Scheme 10), epoxides 30, 32 (Scheme 13), diketones 31, 33 (Scheme 13) and ether-alcohols 35 and 36 (Scheme 14) have been prepared. Most of these conversions show high stereoselectivity.