Dioxaborolane functions

A second powerful route to functionalized allylboron compounds involves the reaction of an a-haloalkylboronatc and a vinyl organometallic reagent3 4-28-29, 50c-92 04. This method is especially useful for the preparation of allylboron compounds not accessible via the allylorganometal-lic route. Notable examples that fall into this category are ( )-4,4,5,5-tctramethyl-2-[4-(tetrahy-dro-2//-pyran-2-yloxy)-2-butenyl]-l,3,2-dioxaborolane (yield 41 %, 93% E) and (E)- or (Z)-2-(l,l-dimethyl-2-butenyl)-4,4,5,5-tetramethyl-1.3,2-dioxaborolane (yield 77-84%. 98% E or 93% Z). 

The enantioselective cyclopropanation leading to 1,2,3-snbstitnted cyclopropane derivatives proceeds with high diastereocontrol (equation 86) . It is quite interesting to observe that the same reaction, when run in the absence of the dioxaborolane ligand, led to lower diastereoselectivity. Other functionalized 1,1-diiodoaLkanes can be used as the zinc carbenoid precursor, but it should be noted that up to 2 equivalents of the reagent (4 equivalents of RCHI2) are needed in this process. This reaction has been applied in the synthesis of ambruticin. ... 

Enantioselective Synthesis of 1,2,3-trisubstituted Cycio propanes. The chiral dioxaborolane ligand can also be used to generate 1,2,3-substituted cyclopropyl units when the appropriate 1,1-diiodoalkane is used in the preparation of the zinc reagent (eq 9). The reaction affords 1,2,3-trisubstituted cyclopropanes with excellent enantio- and diastereocontrol, including those obtained from functionalized zinc reagents (eq 10). 

This method is comparable to similar, catalytic Sim-mons-Smith-type methods employing the titanium TADDOL catalyst 20 (95 5 er) or the Ci-symmetric bis-sulfonamide catalyst 32 (93 7 er) for the cyclopropanation of the allylic alcohol 22 (eq 6). However, due to the preliminary nature of these earlier investigations, substrate scope and generality have not been extensively documented. All of the aforementioned methods are limited by their dependence on the allylic alcohol functionality. Only one method for Simmons-Smith-type cyclopropanation of other substrate classes has been developed. Use of a stoichiometric, chiral dioxaborolane [CAS 161344-85-0] additive allows for selective cyclopropanation of allylic ethers, homo-ally lie alcohols and allylic carbamates. ... 

For example, it has been used to elaborate the chiral cyclopropanes subunits of Curacin A[60], and of the structurally fascinating FR-900848 [61] and U-106305 [62]. The chiral dioxaborolane-derived ligand was also effective to synthesize 1,2,3-substituted cyclopropanes [63]. Excellent to outstanding diastere-oselectivities and enantioselectivities were observed when a variety of allylic alcohols were treated with the reagent formed by mixing 1,1 -diiodoethane and di-ethylzinc. It was also shown that functionalized 1,1-diiodoalkanes could also be used in this reaction. 

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