No abstract

A formal regioselective cross-coupling of various pyridines with alkyl and aryl groups can be achieved by a BF3·OEt2-mediated addition of Grignard or organozinc reagents to pyridines bearing various substituents (chloro, bromo, cyano, vinyl, phenyl, carbethoxy, nitro, etc.) followed by an oxidative aromatization mediated by chloranil. Good regioselectivity and wide functional group tolerance make this method very versatile for the preparation of polyfunctional pyridines. No transition-metal catalyst is required in these coupling reactions.

No abstract

Page 2345. Regretfully, a wrong version of Scheme 91 was inserted in the final manuscript. Please consider the following scheme as the final version of Scheme 91: Related text should read (p 2345, Lines 10-18): To circumvent the aforementioned lack of reactivity, vinyl sulfones 462, 546, and 554 were converted to vinyl phosphonates 548, 549, 555, and 556 in 69-82% yield using Noshi's conditions (Scheme 85; cf. 511 f 516). 161 Interestingly, ozonolysis of 555 and 556 in CH 2 Cl 2 /MeOH does not provide aldehyde-methyl esters 559 and 560 but rather gives the easily isolable aldehyde-acyl phosphonates 557 and 558 in near quantitative yield, consistent with the greater stability of acylphosphonates 169 relative to acyl sulfones. The numbering change caused by the modification above alters the numbering in text and Schemes 92 and 93 as follows. Related text should read (p 2345 Lines 24-27, p 2346 Lines 1-7): For example, conversion of generic bromide 561 to phenyl sulfone 562, C-C bond formation to 563, followed by reductive cleavage to 564 has failed to exploit the pluripotency of the phenylsulfone. The overall operation requires an introduction/ removal of the activating function simply to install a single C-C bond (probably without stereochemical control). This limitation is especially obvious in the 21st century, where synthesis of 564 from 561 would likely be achieved by a single metalcatalyzed stereocontrolled operation (Scheme 92). Related text should read (p 2346, Lines 8-10):The cumulative power of the sulfur functionality is nicely illustrated in the synthesis of enantiopure lactone 567 (Scheme 93).

A Jacobsen epoxidation protocol using H2O2 as oxidant was designed for the large-scale preparation of various epoxy vinyl sulfones. A number of cocatalysts were screened, and pH control led to increased reaction rate, higher turnover number, and improved reliability.

Starting from an appropriate 6-chloro-2-TMS-purine derivative, a regioselective functionalization of the purine scaffold was achieved successively at positions 8, 6, and 2 via zinc and magnesium intermediates which were generated either by a direct zincation with TMPZnCl·LiCl or by an I/Mg exchange with iPrMgCl.

Enantiopure cycloheptadienyl sulfones 6 and 7 are diastereoselectively epoxidized to yield epoxyvinyl sulfones 8, 9, 14, and 16 in high yields and diastereomeric ratios. Syn and anti methylation of epoxides 8, 9, 14, and 16 enables access to all eight possible diastereomeric stereotetrads, seven of which are commonly found in polypropionate natural products. Anti methylations of the above epoxides are possible by either the reaction of methyl organometallics promoted by copper(I), or via reaction with trimethylaluminum to yield stereotetrads 11, 12, 22, and 24. Syn methylations are achieved via Lawton SN2' reaction in the case of stereotetrads 10, 15, and 38, while stereotetrad 13 is accessed by an oxidation/reduction alcohol inversion sequence from stereotetrad 11. All stereotetrads were obtained in high diastereomeric ratios and yields, and their relative stereochemistry was confirmed by X-ray crystallography. Oxidative cleavage of the cyclic stereotetrads yields termini-differentiated acyclic heptanyl stereotetrads ready for use in building larger fragments in the course of target syntheses.

To apply our collection of enantiopure 7-ring vinyl sulfones to probe the anticancer SAR of a series of computer-designed (+)-discodermolide analogs, the ozonolytic reactivity of transposed cyclic vinyl phosphonates was explored. Successful preparation of the desired aldehyde-esters and lactones from vinyl phosphonates via an oxidative cleavage-phosphite/methanol exchange sequence is described.