Vinylmagnesium bromide 1 : 4 addition

Vinylmagnesium bromide addition to 509 results in a tertiary alcohol that upon treatment with phosphorus tribromide undergoes rearrangement to furnish a 65% yield of the -allylic bromide 510 (along with 25% Z-bromide). This is coupled with the anion of 511 to afford 512, which is elaborated to the antibiotic 513 [162,163]. 

Cyclopentane and cyclohexane compounds have been prepared by employing chelation-controlled stereoselective vinylmagnesium bromide additions to aldose sugars, and subsequent construction of the carbocyclic rings by intramolecular nitrone cyclization using Bernet and Vasella s methodology as illustrated for the amino-... 

Finally (d + aV dditions of 1-alkenyl and 1-alkynyl anions to carbonyl groups should be mentioned. Examples are the addition of vinylmagnesium bromide to ketones e.g. in the first step of Torgov s steroid synthesis (I.N. Nazarov, 1957), and the famous alkynylation of... 

The addition of vinylmagnesium bromide to methyl (S)-3-benzyloxy-4-oxobutanoate (5) in tetrahydrofuran proceeded with a slight preference for the nonchelation-controlled reaction product (40 60)5°. A reversal of the diastereoselectivity (80 20) could be observed when the Grignard reagent, as a solution in tetrahydrofuran, was added to a dichloromethane solution of the aldehyde which had been precomplexed with one equivalent of magnesium bromide. The almost exclusive formation of the chelation-controlled reaction product 6 was achieved when tetrahydrofuran was completely substituted by dichloromethane the presence of tetrahydrofuran interferes with the formation of the chelate complex, which is a prerequisite for high chelation-controlled diastereoselection. 

Whereas the nucleophilic addition of vinylmagnesium bromide to a-alkoxy aldehydes (12, 16) proceeds with a low to moderate chelation-controlled diastereoselectivity, a remarkably high preference for the opposite stereochemical behavior is found with the jS-silyl phosphorus ylide 1477. Due to the electron-donating 4-methoxyphenyl substituents at the phosphorus atom, as well as the /i-methyldiphenylsilyl group, 14 is an excellent vinylation reagent which does not lead to any Wittig olefination products. 

As with oxathianes 3 (R1 = CH, R2 = H), which bear a close structural resemblance to 17, the addition of organometallic reagents is highly diastereoselective with a predominant chelation-controlled attack of the nucleophile from the Rc-sidc35 -40. In the case of vinylmagnesium bromide a considerable enhancement of the diastereo selectivity could be attained by adding... 

High stereoselective addition of vinylmagnesium bromide to L-tartaric acid-derived nitrone was used as a key step in the synthesis of (+ )-lentiginosine and its structural analogs (653). 

The configurational assignment thus rests solely on the stereochemical course of the copper-catalyzed conjugate addition of vinylmagnesium bromide to 7. a reaction process known to occur with trans stereoselectivity 106. 

Addition of vinylmagnesium bromide 5 to N-monoprotected a-amino aldehydes 4 usually yielded a mixture of diastereoisomers, with a preference for the chelation-controlled... 

An approach to the synthesis of nonaromatic steroids involves conjugate addition of vinylmagnesium bromide to 2. In this reaction, prior addition of zinc bromide to... 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

---