Homoallylic alcoholates, fragmentation

The addition of substituted allylic zinc reagents to aldehydes is usually unselective" . Furthermore, the direct zinc insertion to substituted allylic halides is complicated by radical homocoupling reactions. Both of these problems are solved by the fragmentation of homoallylic alcohols. Thus, the ketone 166 reacts with BuLi providing a lithium alcoholate which, after the addition of ZnCl2 and an aldehyde, provides the expected addition product... 

Fragmentation of homoallylic aikoxides.3 Thermolysis of homoallylic alcohols requires temperatures >200°. However, fragmentation of bicyclo[2.2.2]-5-octene-2-alkoxides (1) prepared as shown in equation (I), occurs at 90-120° and results mainly in unsaturated ketones (2) formed by cleavage of the allylic (C -C2) bond. Although the exo-alkoxide is cleaved noticeably faster than the endo-cpimer, both aikoxides give the same product. 

In order to apply tartrate ester-modified allyl- and crotylboronates to synthetic problems,23 Roush and Palkowitz undertook the stereoselective synthesis of the C19-C29 fragment 48 of rifamycin S, a well-known member of the ansamycin antibiotic group24 (Scheme 3.1u). The synthesis started with the reaction of (S,S)-43E and the chiral aldehyde (S)-49. This crotylboration provided the homoallylic alcohol 50 as the major component of an 88 11 1 mixture. Compound 50 was transformed smoothly into the aldehyde 51, which served as the substrate for the second crotylboration reaction. The alcohol 52 was obtained in 71% yield and with 98% diastereoselectivity. After a series of standard functional group manipulations, the alcohol 53 was oxidized to the corresponding aldehyde and underwent the third crotylboronate addition, which resulted in a 95 5 mixture... 

Allyliczinc halides reagents have been prepared from sterically hindered homoallylic alcohols, using a novel fragmentation reaction of the corresponding zinc aUcoxide (Scheme 6). The interest of this reaction resides in the absence of the Wurtz homocoupling product, usually present in high proportion when the direct reaction of zinc with allylic halides is used. 

Prochiral a-(acylamino)acrylic acids or esters are hydrogenated under an initial hydrogen pressure of 3-4 atm to give the protected amino acids in up to 100% ee (eq 16). The BINAP-Rh catalyst was used for highly diastereoselective hydrogenation of a chiral homoallylic alcohol to give a fragment of the ionophore... 

The modest Lewis acid properties of Al(OPr )3 are needed for the fragmentation reactions of epoxides derived from homoallylic alcohols described recently by Waddell and Ross. Carbonyl groups in the initial fragmentation products are reduced by Al(OPti)3 to give the materials shown in equations (138) and (139). 

The synthesis of the C1-C7 fragment, which corresponds to the lactone, starts with the homoallylic alcohol 2 which was prepared from 1. The existing stereocenter and the conjugate addition method of Evans [21] allow the control of the C5 stereocenter. The homoallylic alcohol 2 was oxidatively cleaved and homologated to the trans enoate 3 by a Wittig olefination. Treatment of 3 with benzaldehyde and a catalytic amount of KHMDS provided acetal 4. The internal Michael addition of the hemiacetal intermediate proceeds with complete stereoselectivity [22]. After deprotection and oxidation, the corresponding aldehyde was treated with Amberlyst-15 and then with camphor sulfonic acid (CSA), to yield pyrane 5 as a mixture of (3- and a-anomers (1.8/1). This compound was converted to the thiophenyl acetal 6 (4 steps) as this compound can be hydrolyzed later under mild conditions (Hg +) with subsequent oxidation of the lactol to the desired lactone. Compound 6 represents the C1-C7 fragment of discodermolide (Scheme 1). 

The C16-C24 ketone fragment 11 was prepared from the homoallylic alcohol 2. After protection of 2 as a p-methoxybenzyl (PMB) ether [24], the oxidative cleavage led to an intermediate aldehyde which was converted to the cis vinyl iodide 9 by using the... 

Fragmentation. Cyclobutyl sulfides in which an epoxide or oxetane ring is attached to the adjacent carbon atom are liable to fragmentation. Both rings undergo cleavage, leading to allylic and homoallylic alcohols, respectively. 

Tertiary alcohols are prone to fragmentation this process is facrhtated by a -trimethylsilyl group (eq 4). Other alcohols prone to Iragmentation are cyclobutanols, strained bicyclo [x.y.z]alkan-2-ols, and homoallylic alcohols. ... 

Secolongifolene diol is one of the metabolites of the fungus Helminthosporium sativum (725). Recently, a simple synthesis of its optical antipode (193) has been achieved by exploiting the electrophilic addition-fragmentation reaction (726) of homoallylic alcohols. The homoallylic alcohol (191), which can be obtained from longicyclene (44) by cyclo-... 

Yadav, j. S., H. P. S. Chawla, and Sukh Dev Cleavage of Homoallylic Alcohols. A Novel Fragmentation Reaction. Tetrahedron Letters 1977, 201. 

---