Cycloaddition reactions of bridgehead enones

Abstract

The cycloaddition reactions of bridgehead enones derived in situ from ketones 8,9, and 10 with various dienes at 0 "C afford good yields of adducta. Even 1,1,3-trisubstituted dienes work well. The exclusive exo stereochemistry can be rationalized in terms of a stepwise mechanism involving ionic intermediates. Disciplines 1397solution was stirred at -78 "C for 4 h, quenched with 0.3 mL of saturated sodium bicarbonate solution, diluted with ethyl acetate, dried, and concentrated in vacuo. Benzene was added and the solution was again concentrated in vacuo. The residue was dissolved in methylene chloride (10 mL) and cooled to 0 "C. Pyridine (0.5 mL), acetic anhydride (0.15 mL), and DMAP (1 mg) were then added and the solution was allowed to slowly warm to room temperature overnight. The solution was concentrated in vacuo, ether (30 mL) was added, and the organic layer was extracted with copper sulfate solution. The organic layer was dried, concentrated in vacuo, and purified by chromatography using hexaneslethyl acetate. The purification afforded 9 mg (55% yield).

Diels−Alder Exo Selectivity in Terminal-Substituted Dienes and Dienophiles: Experimental Discoveries and Computational Explanations

“…During the study of the preparation of enantioenriched fluorinated carbocycles, we synthesized an array of Diels−Alder cycloadducts featuring an allylsilane or a silyl enol ether, starting from all-carbon silylated dienes or silyloxy dienes, respectively (Scheme ). , We observed a remarkable change in the endo/exo stereoselectivity of the cycloaddition that depends on the substitution pattern of the starting material. As depicted in Scheme , when both R 1 and R 2 are a methyl group (Scheme , right), the adducts with four stereocenters are formed with a consistently high exo selectivity.…”

Synthesis of 2-Azabicyclo[3.3.1]nonanes

“…Kraus synthesized morphan 196 while studying bridgehead enone formation (Scheme 77). 138 The ketal amine was converted into an imine, which was treated with the sodium salt of ethyl acetoacetate and then protected as a carbamate. Ketal hydrolysis in 195 followed by an intramolecular aldol afforded morphan 196.…”