Allyl Brown reagent

Results of reactions of chiral a-methyl aldehydes and several chiral crotyl- and allyl-boron reagents are summarized in Tables 8 and 9. It is apparent from these data that the Brown (Ipc)2B(crotyl) and (Ipc)2B(allyl) reagents (51), (52) and (219) consistently give excellent results for the synthesis of each product diastereomer (Table 8, entries 3-6, 11, 16, 20, and 24 Table 9, entries 1,2, 10 and 18). This is true also for their reactions with chiral a- and 3-alkoxy aldehydes (Scheme 49).i. i4S-i50 Thg tartrate crotylboronates (18) and (19) also display excellent selectivity in the synthesis of crotyl diastereomers (136), (137) and (139) (Table 8, entries 7,10,13,17,25 and 28), but are much less selective for the syndesis of crotyl diastereomer (138), especially from -alkoxy-substituted aldehydes such as (253). Tartrate allylboronate (224) is also less effective than (Ipc)2Ballyl (219) for the synthesis of (257) and (258) in Table 9, and of (266) and (267) in Scheme 49.Substantial improvements in selectivity have been realized by using the taitramide-based allylboronate (228), and the results with this reagent (Table 9, entries 4, 7, 9, 12, 14, 17, 20 and 22) compare very favorably with those obtained with (219). The data... 

For reviews, see Hoffmann, R.W. Niel, G. Schlapbach, A. Pure Appl. Chem., 1990, 62 1993 Pelter, A. Smith, K. Brown, H.C. Borane Reagents Academic Press NY, 1988, p 310. For a review of allylic boranes, see Bubnov, Yu.N. Pure Appl. Chem., 1987, 21, 89, For an example that proceeds with asymmetric induction, see Buynak, J.D. Geng, B Uang, S. Strickland, J.B. Tetrahedron Lett., 1994, 35, 985. 

Generation and Reaction of Allyltitanium Reagents (Section 9.3) 2-(4-Bromophenyl)-l-phenyl-3-buten-l-ol [42] To a solution of l-(4-bromophenyl)allyl ethyl carbonate (285 mg, 1.0 mmol) and Ti(OiPr)4 (0.296 mL, 1.0 mmol) in diethyl ether (5 mL) was added iPrMgBr (1.20 m in diethyl ether, 2.0 mmol) at 50 °C. The resulting yellow solution was stirred at —50 to —40 °C for 1.5 h, in the course of which it became brown. Benzaldehyde (74.3 mg, 0.70 mmol) was then added at —40 °C and the mixture was allowed to warm to 0 °C over a period of 30 min. After the addition of aqueous 1 n HC1 (5 mL) at this temperature, the mixture was allowed to warm to ambient temperature. The organic layer was separated and the aqueous layer was extracted with diethyl ether (10 mL). The combined organic layers were washed with saturated aqueous NaHC03 solution (5 mL), dried over... 

Chiral Dialkylboranes. Several allylic boranes have been developed as chiral auxiliary reagents (Fig. 5). The introduction of terpene-based reagents such as 12 and 64-68 has been pioneered by H.C. Brown, and the most popular class remains the bis(isopinocampheyl) derivatives (structures 12, 64-66). A wide variety of substituted analogs have been reported, including the popular crotylboranes but also a number of other reagents bearing heteroatom-... 

For reviews, see Hoffmann Niel Schlapbaeh Pure Appl. Chem. 1990, 62, 1993-1998 Pelter Smith Brown Borane Reagents-, Academic Press New York, 1988, pp. 310-318. For a review of allylic boranes, see Bubnov Pure Appl. Chem. 1987, 21, 895-906. 

Brown and co-workers developed a novel homoallenyl boronate reagent 169 based on diisopropyl tartrate for the stereoselective homoallenylation of aldehydes 170. The reagent 169 was prepared via homologation of the corresponding allenyl boronate or the alkylation of halomethyl boronate with allenyl Grignard similar to those reported in Scheme 26. The allyl boronate 169 upon reaction with aldehydes furnished the dienyl alcohols 172 with high ee (Scheme 28) <1996JOC100>. 

In the synthesis of a library of (+)-murisolin (39) and its 15 other stereoisomers, Curran and others used Brown allylation strategy to obtain the four diastere-oisomers of the homoallylic alcohol 4019 (Scheme 3.1p). Thus, the homoallylic alcohol (S,S )-40 was prepared in 95% ee from allylborane reagent l35 and corresponding aldehyde. By using the Mitsunobu reaction, (R,S )-40 was obtained. Similarly, (R,R)-40 was synthesized via the enantiomeric borane reagent d35... 

Scheme 11-10). It should be noted that glyeeraldehyde derivatives are outstanding substrates for the tartrate ester-modified allylboronates [118]. Aldehyde 261, derived from 260 in two steps, underwent a highly stereoselective (selectivity = >99 1) allylation reaction with the Brown Ipc2BAllyl reagent 195 [112, 113] (an in-depth discussion of the synthesis and use of this reagent appears in Section... 

From the range of synthetic applications of Brown s pinene-derived allyl- and crotylborane reagents illustrated in these selected examples, it is clear that they are excellent reagents for the synthesis of chiral homoallylic alcohols from chiral and achiral aldehydes alike. That so many researchers have applied these reagents in their research also attests to the great utility of these reagents in organic synthesis. 

Using Keck s original catalytic allylation procedure, Danishefsky and co-workers converted aldehyde 474 to the homoallylic alcohol 475 (conditions A, Scheme 11-38, 60% yield, >95% ee) used in their total synthesis of epothilones A and B [314], Asymmetric allylation with a stoichiometric amount of Brown s reagent, [(-)-lpc]2BAll (195), however, was higher yielding and required a shorter reaction time (conditions B, Scheme 11-38, 83% yield, >95% ee). 

The enantioselective asymmetric allylation of imines has been a synthetic challenge, the initial solutions of which required stoichiometric amounts of chiral allylbor on [87], allylsilane [88], allylzinc [89], or allylindium reagents [90]. Itsuno showed that a chiral B allyloxazaborolidine derived from norephedrine could add to the N trimethylsilyl imine prepared from benzaldehyde in high yield and enantiomeric excess (Scheme 1.22) [91]. Brown later reported that B allyldiisopinocamphenylbor ane is also very effective for the allylation of the same electrophiles, but the addition of a molar amount of water is necessary to obtain high yields [92]. The diastereo and... 

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