Studies on the Mercury-Desilylation of Chiral Cyclopropylmethylsilanes - A Stereocontrolled Access to Carba-Sugars

Development of domino processes by using 7-silylcycloheptatrienes and its analogues

7-Silyl- and 7-silylmethylcycloheptatrienes were shown to react with acylnitroso reagents at room temperature, through their norcaradiene forms, to generate the corresponding cycloadducts 5 a-b and 6 a-b as single diastereomers. The course of the reaction was dramatically modified by changing the reaction conditions. Using a polar medium, functionalized cyclohexa-1,3-dienes 7 a-b and bicyclic compounds 13 a-b were instead generated, incorporating one or two amino groups. Similar behavior was observed by using other dienophiles, including triazolinedione, but also activated aldehydes and ketones. A tentative mechanism has been proposed to rationalize the formation of both classes of products that relies on a domino process involving four consecutive elementary steps, in this order: 1) electrocyclic process, 2) hetero-Diels-Alder reaction, 3) cyclopropane ring opening, and 4) hetero-Diels-Alder reaction. Trapping of the cationic intermediate and isolation of the primary cycloadduct provide support for this hypothesis. An enantioselective version of the cascade using cycloheptatriene 4 b and aldehydes and ketones, under copper(II) catalysis was also carried out, leading to cyclohexa-1,3-dienes 21, 28, and 30 with enantioselectivities up to 93 % ee. Finally, elaboration of the intermediates above has been carried out, opening a straightforward access to sugar mimics 42-43 and complex polycyclic systems 36 and 39.

A practical, convergent route to the key precursor to the tetracycline antibiotics

Here we describe a 5-step sequence to prepare the AB enone 1, the key precursor to fully synthetic tetracyclines, that begins with a diastereoselective Michael-Claisen coupling of two simple starting materials, a cyclohexenone (compound 2 or, in a refinement, a substituted variant, vide infra) and the isoxazole ester 3. This advance defines an 8-step linear sequence to 6-deoxytetracycline antibiotics from three components of similar complexity (cyclohexenone 2, isoxazole ester 3, and structurally diverse D-ring precursors) in which sequential diastereoselective Michael-Claisen cyclization reactions form the A- and C-rings, respectively, of the linearly fused ABCD tetracycline skeleton. In addition to providing a readily scalable, practical route to fully synthetic tetracyclines of broad structural diversity, the sequence reported comprises a series of non-obvious stereoselective transformations, including a novel means for C12a hydroxylation.

Ring scission of diastereomeric 4-butylspiropentylcarbinyl radicals as a chemical model for identifying enzyme-catalyzed FAD adducts resulting from spiropentylacetyl-CoA

Both diastereomeric 4-butylspiropentylcarbinyl bromides (14a and 14b) were synthesized in seven steps starting from 1-heptyne, and the stereochemical assignments based upon NOE experiments were confirmed by converting their immediate alcohol precursors (13a and 13b) to 1,4-dibutylspiropentanes (17a and 17b) with C(1) and C(2) symmetry. Each bromide was used to generate its corresponding spiropentylcarbinyl radical (18a and 18b) via its AIBN-initiated tri-n-butyltin hydride reduction. The radical-trapped products are identified, the preferred ring scission mode is identified (C1[bond] C2 bond cleavage), and the estimated rates for the ring opening of 4-butylspiropentylcarbinyl radical (18, k(25) degrees C > or = approximately 5 x 10(9) s(-1)) and 2-butyl-1-vinylcyclopropylcarbinyl radical (33, k(25) degrees C approximately 5 x 10(8) s(-1)) are reported. High-level ab initio calculations addressing the ring-opening isomerizations of cyclopropylcarbinyl and spiropentylcarbinyl radicals also are presented. These results in conjunction with a previous study enable us to propose two structures for the enzyme-catalyzed FAD adducts resulting from spiropentylacetic acid-CoA, a synthetic byproduct of fatty acid metabolism.