SPIROLACTONES FROM DIRHODIUM(II)-CATALYZED DIAZO DECOMPOSITION WITH REGIOSELECTIVE CARBON-HYDROGEN INSERTION

Dirhodium(II) caprolactamate, Rh-2(cap)(4), catalyzes diazo decomposition of cycloalkylmethyl diazoacetates which form spirolactones in moderate to high yield by insertion into a tertiary carbon-hydrogen bond. Similar results are obtained with diazoacetates derived from tetrahydropyran-2-methanol and tetrahydrofurfuryl alcohol but not from cyclopropylmethanol. With tetrahydrofuran-3-ylmethyl diazoacetate, Rh-2(cap)(4) catalysis promotes delta-lactone formation via insertion into the oxygen-activated secondary C-H bond instead of gamma-lactone formation by carbene insertion into the unactivated tertiary C-H bond. However, when both 1,5- and 1,6-positions are activated for insertion by adjacent oxygen atoms, as in (2,2-dimethyl-1,3-dioxolan-4-yl)methyl diazoacetate, five-membered ring formation occurs exclusively in Rh-2(cap)(4)-catalyzed reactions, whereas use of dirhodium(II) acetate leads to both insertion products.