Scheme 19 Dissected cascade rearrangements with 2-lithiodihydrofuran as an alkenyl anion [103,104]

A different approach to triquinane was made by Moore [107-110]. His synthetic route includes the intramolecular [2 + 2] cycloaddition of the vinylketene intermediate as shown in Fig. 4. If the bicyclo[3.2.0]heptenone from this reaction is designed to have an alkenyl-substituent at the bridgehead (i.e., 101 ^ 102 ^ 103), the next oxy-Cope rearrangement is satisfied by adding another alkenyl group (i.e., 104) to give the triquinane 106 (Scheme 20).

As a matter of fact, the above preparative reaction to obtain the framework of bicyclo[3.2.0]heptenone is already in hand. Indeed, the ring closure step after electrocyclic ring opening of 4-hydroxy-2-cyclobutenone is not limited to fully conjugated systems; synthetic variants are realizable with other prox-imally placed ketenophiles. When an allyl group was located at C-4, the ketene underwent an intramolecular [2 + 2] cycloaddition reaction with this double bond to give the bicyclo[3.2.0]heptenone derivatives [111,112].

Scheme 20 Different route to triquinane via oxy-Cope rearrangement of bicyclo[3.2.0] heptenone [107-110]

While an allylic portion has hitherto been introduced under usual nucleo-philic conditions, allylsilanes 108 are the reagent of choice as an alternative under electrophilic conditions. The electrophilic center was generated from cyclobutenedione monoacetal 107 with BF3 catalyst, being allowed to react smoothly to give regiospecifically allylated product 109. This was obtained as a protected form and utilized directly for the following thermal ring opening to give the expected [2 + 2] cycloadducts 111 in a high yield. A triquinane framework 112 was also accessible by this route from one-carbon homologation of the adduct with diazoacetate, when cy-clopentylmethyltrimethylsilane (108, R1 - R2 = CH2CH2CH2) was employed as a starting reagent. A tricyclic oxygen-heterocycle 114 was constructed by the same sequential reactions using 6-hydroxy-2-hexenyltrimethylsiane (108, R1 = H, R2 = CH2CH2CH2OH) [42] (Scheme 21). Interestingly, reactivity of cyclobutenones having both phenyl and allyl groups at C-4 (obtained by BF3-catalyzed reaction of 4-phenyl-4-hydroxycyclobutenone with allylsilane) was judged to be competitive between the thermal [2 + 2] cycloaddition and 6n-electrocyclic ring closure under equilibrated conditions (cyclobutenone ^ vinylketene), although inward rotation is preferred for the allyl substituent on the basis of torquoselectivity arguments [43].

The 2-chloro-4-hydroxy-2-cyclobutenone with an acylmethyl substituent at C-4 (115) was available from the electrophilic reaction of ester chloride 18 with silyl enol ether and silyl ketene acetal (Scheme 3). This was also found to be thermolabile to give a rearranged product, y-acylmethylenetetronate 118 [113]. In this case, the cyclization occurred by choosing the hydroxyl function as a proximal ketenophile from an equilibrated mixture. Although the [2 + 2] cycloaddition mode might be a possible route to a ^-lactone according to the favored outward rotation of a hydroxyl group (115 ^ 116a), the equilibrium could be shifted by lactonization and dehydrochlorination to thermodynamically stable (Z)-tetronate (116b ^ 117 ^ 118) (Scheme 22). Photochemistry of the same compound resulted in the formation of chlorine-





HO OB R2 109

P R1

0 0

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