Rearrangements of cyclobutenones. Electrocyclic ring closure and thermal ring expansions of 3-allenyl- and 3-alkynyl-2-dienyl-4,4-dimethoxycyclobutenones.

Thermal rearrangements of 2-allenyl- and 2-alkynyl-3-(2-ethenylphenyl)-4,4-dimethoxycyclobutenones were studied. At ambient temperature, the allenyl compounds undergo an electrocyclic cascade to give bicyclo[4.2.0]octadienyl-fused cyclobutenones. These unusual tetracyclic cyclobutenones were shown to be viable synthetic precursors to benzo[a]anthracene-7,12-diones, compounds representing the framework of the angucycline group of naturally occurring antibiotics. In contrast, the 2-alkynylcyclobutenones are stable at ambient temperature but undergo a facile rearrangement at 110 degrees C (toluene) to give the previously unknown naphthalene derivatives, 1,2-dihydro-2,2-dimethoxy-1-(3-alkenylidene)naphtho[2,1-b]furans.

Cyclobutenone-Based Syntheses of Polyquinanes and Bicyclo[6.3.0]undecanes by Tandem Anionic Oxy-Cope Reactions. Total Synthesis of (+/-)-Precapnelladiene.

The addition of ethenyllithium derivatives to the carbonyl of dialkyl squarate-derived bicycloheptenones, e.g., 1a and 6a, initiates a low-temperature anion-accelerated oxy-Cope rearrangement to provide polyquinanes by a transannular aldol reaction of the intermediate bicyclo[6.3.0]undecadienone 4. Additional functionality is introduced by alkylation of the enolate 3 resulting from the oxy-Cope rearrangement. Phosphorylation or triflation of enolate 3 provides an entry into the bicyclo[6.3.0]undecane ring system. An application of this new methodology is demonstrated by the total synthesis of the sesquiterpene natural product (+/-)-precapnelladiene from diisopropyl squarate (10 steps, 12%).