Cyclic conjugated enediynes via elimination of a thionocarbonate in a latent Z-hex-3-ene-1,5-diyne unit

Application of photochemical decarbonylation of cyclopropenones for the in situ generation of reactive enediynes. Construction of a cyclopropenone-containing enediyne precursor by using a cyclopropenone acetal building block

[reaction: see text] A cyclopropenone-containing enediyne photoprecursor, 6-tert-butyldimethylsilyloxy-3-(alpha-hydroxybenzyl)bicyclo[8.1.0]undeca-1(10),2-diene-4-yn-11-one (1), was prepared in 10 steps by sequential modification of the cyclopropenone 2,2-dimethyl-1,3-propanediyl acetal (5). The crucial cyclization step was achieved under Nozaki conditions, while the endocylic double bond has been introduced by the allylic rearrangement. UV irradiation of the cyclopropenone 1 results in efficient decarbonylation and the formation of the reactive enediyne 2. The latter undergoes Bergman cycloaromatization with a half-life of 12 h at 40 degrees C and in the presence of 1,4-dihydrobenzene quantitatively produces corresponding tetralin 29.

Enantiospecific synthesis of the (9S,18R)-diastereomer of the leukocyte adhesion inhibitor cyclamenol A

Cyclamenol A is one of the very few non-carbohydrate and non-peptide natural products that inhibit leukocyte adhesion to endothelial cells. We report on the first enantioselective total synthesis of the (9S, 18R)-diastereomer of this macrocyclic polyene lactam. Key elements of the synthesis are i) the synthesis of the required chiral building blocks by employing readily accessible building blocks from the chiral pool, that is, (S)-malic acid and (R)-hydroxyisobutyric acid, ii) assembly of a linear polyene precursor by means of Wittig and Horner olefination reactions as key C-C bond-forming transformations, iii) ring closure by means of a vanadium-mediated pinacolisation reaction and iv) conversion of the generated cis-diol into a (Z)-olefin to complete the entire polyene system of the natural product. Attempts to close the macrocyclic ring by a macrolactamisation, a double Stille coupling or direct olefination in a McMurry reaction failed. Crucial to the successful completion of the synthesis was the correct orchestration of the final steps. It was necessary to first deprotect the intermediate formed after macrocycle formation and to generate the sensitive heptaene system in the last step by means of a Corey-Hopkins sequence.

Electronic control of the Bergman cyclization: the remarkable role of vinyl substitution

We report an ab initio study of the effect of vinyl substitution on the cycloaromatization of 3-ene-1,5-diynes (the Bergman cyclization). The majority of the calculations were conducted by using the BLYP version of Density Functional Theory, and higher level Brueckner orbital calculations were used for a few key compounds. In all, 46 enediynes, 44 cyclization transition states, 39 singlet p-benzynes, and 28 related triplet p-benzynes were studied, including simple vinyl-substituted and annulated examples. The data indicate that strongly electron-withdrawing groups increase the cyclization barrier, while sigma-donating groups decrease it; pi conjugation, especially donation, has little effect. Most annulations, including those involving heteroaromatic rings, lower the barrier slightly (6 MR) or raise it slightly (5 MR). Larger effects are seen for smaller rings or charged rings. Some previously observed apparent rate inhibitions are seen to be due to reversibility or forward reactivity of the intermediate p-benzynes, which are thereby inhibited from the H abstraction step that completes cycloaromatization. H abstraction reactivity, as judged from the p-benzyne singlet-triplet energy gap and from isodesmic equations, is also examined. Unexpected behavior is predicted for some heteroaromatic systems. Finally, we anticipate how these results may be applied to the design of prodrug candidates for subsequent biological application.