Photooxygenation of the helimers of (-)-isocolchicine: regio- and facial selectivity of the [4 + 2] cycloaddition with singlet oxygen and surprising endoperoxide transformations

Photooxygenation of the helimeric mixture of (-)-(M,7S)/(P,7S)-isocolchicine (6) with the superdienophile singlet oxygen has been studied. Cycloaddition occurred with high regioselectivity at the 7a,11-positions of the alkaloid and predominantly at the diene face anti to the amidic substituent at the stereogenic center C-7, leading to two endoperoxides 7 (syn) and 8 (anti) with an 1:7 ratio. The structure of the minor product 7 was established by X-ray analysis. Investigation of the triethylamine induced transformation of the predominant endoperoxide 8 furnished a mixture of two isomers (M,7S)-10a/(M,7S)-10b in a 2:1 ratio possibly with constitutional interconversion and with (M,7S)-9 as plausible intermediate. Treatment of this mixture with silica gel/ethyl acetate at ambient temperature surprisingly led to an atropenantiomerically pure colchicinoid (M,7S)-12 characterized by an eightmembered oxocine B-ring, the structure and absolute configuration of which could be determined by X-ray analysis. For the unprecedented formation of the novel colchicinoid (M,7S)-12 a plausible reaction pathway is suggested, involving a complete transfer of the (M) helical asymmetry of the intermediate (M)-11 into (S) asymmetry of the newly formed carbon center of (M,7S)-12. Prerequisite for such a scenario is the configurational stability of the intermediate pseudobiaryl (M)-11, under the conditions employed, allowing to transmit the axial chirality onto the chiral center of the product (M,7S)-12.

Bis(2,2'-bipyridine)(dicyanamido)copper(II) tetrafluoroborate

In the title compound, [Cu(C(2)N(3))(C(10)H(8)N(2))(2)]BF(4), the Cu(II) atom shows distorted trigonal-bipyramidal geometry, with the dicyanamido ligand in the equatorial plane. The two out-of-plane Cu-N bond lengths to bipyridine are 2.006 (3) and 1.998 (3) A, whereas the in-plane Cu-N distances are 2.142 (3) and 2.043 (3) A to the bipyridine, and 2.015 (3) A to the dicyanamide.

A Five-Membered Ring with Three Negative Charges and Solvent-Free Lithium Counterions

Remarkably short distances to the ring plane are shown by the eta(5)-bound lithium ions in the first compound with a triply negatively charged five-membered ring, 1, which was obtained by reduction of 2 with lithium. R=CH(SiMe(3))(2), Dur=2,3,5,6-tetramethylphenyl.