Total Synthesis of Dictamnol

The rhodium-cataiyzed intramoiecuiar [5+2] cycioaddition of an allene and vinylcyclopropane was the key step in the asymmetric total synthesis of the trinorguaiane sesquiterpene (+)-dictamnol by P.A. Wender and co-workers.The cyclization precursor allene-cyclopropane was assembled starting from commercially available cyclopropane-carbaldehyde. Using the HWE oiefination, the Weinreb s amide moiety was installed and subsequently reacted with a primary alkyllithium that was generated via lithium-halogen exchange. 

Cycloaddition reactions of cyclopropanes via cleavage of the C—C bond have been apphed to the synthesis of a wide variety of natural products, particularly terpenoids. For example, (-H)-dictamnol [5], (-H)-aphanamol I [6], (-H)-tremulenolide A [7], and (-l-)-lirondosin A [8] were synthesized through a [5-h2] cycloaddition reaction of vinylcyclopropanes with C—C unsaturated bonds. ( )-Hirsutene [9], ( )-pentalenene [10], ( )-asterisca-3(15),6-diene [10], ( )-hirsutic acid [11], and (-l-)-asteriscanohde [12] were synthesized via rhodium(l)-catalyzed [5-H2-H1] cycloaddition of vinylcyclopropanes with alkenes and carbon monoxide. Total syntheses of ( )-a-agarofuran [13], and ( )-pyrovellerolactone [14] were also achieved through cycloaddition reactions of cyclopropane derivatives. For these syntheses, reference to the papers should be made directly. 

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