ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Bromination of the monosilylated dialkynylated monomer 1a (Scheme 1), dimer 3 and tetramer 5 by N-bromosuccinimide (NBS) in the presence of CF3COOAg gave 2, 4, and 6, respectively, in over 93%. Similar conditions led to bromodesilylation. Either silyl group of the diprotected monomer 1c was selectively removed by bromolysis. On the one hand, bromodesilylation of 1c gave 2 in yields varying between 80 and 99%. On the other hand, bromodesilylation of 7, obtained from 1c by hydrolytic removal of the tetrahydro-2H-pyran-2-yl (Thp) group, yielded 91% of 8. Mechanistic considerations suggested that the deprotective bromination should be improved by replacing the Me3Si by a Me3Ge group. Indeed, bromodegermylation of 1b was quantitative and ca. 60 times faster than bromodesilylation of 1c. The Me3Si and Me3Ge groups can be used for an orthogonal protection/activation of dialkynes. This was shown by desilylating 12 to 11 (Scheme 2), while bromination yielded 13. Both reactions proceeded in high yields; 9 was isolated as a minor by-product of 13. The reactivity towards bromolysis decreases in the series H-DOPS 〉 Me3Ge ≈ H 〉 Me3Si 〉 Thp-DOPS (DOPS = [dimethyl(oxy)-p ropyl]dimethylsilyl). Orthogonal bromolysis of DOPS- and Me3Ge-substituted dialkynes is slightly more selective than the one of Me3Si- and Me3Ge-substituted analogues. Coupling of 7 with the bromoalkyne 2 gave the dimer 15 (76%), 14 (2%), and 16 (4%) (Scheme 3). The binomial synthesis was optimized so that each cycle, doubling the size of the precursor, requires the minimal number of transformations (Scheme 4). The orthogonally protected monomer 1b, dimer 19, and tetramer 22 were, on the one hand, hydrolyzed to the alcohols 18 (95%), 21 (91%), and 24(91%), respectively, and, on the other hand, bromodegermylated to 2 (99%), 4 (97%), and 6(93%). Cross-coupling of 18 with 2, 21 with 4, and 24 with 6 gave the orthogonally protected dimer 19 (73%), tetramer 22 (87%), and octamer 25 (83%), respectively.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19960790126
Permalink
