Stereoselective allyl rearrangement

The stereoselective allylic rearrangement of the allylic alcohol 798 catalyzed by PdCl2(MeCN)2 and Ph3P under Mitsunobu inversion conditions is explained as proceeding via a rr-allylpalladium intermediate[496]. The smooth rearrangement of the allylic p-tolylsulfone 799 via a rr-allylpalladium intermediate is catalyzed by a Pd(0) catalyst[497]. 

Stereoselective allylic rearrangement. 2-5 ubstituted I - vinylcyclohexyl acetates (1) rearrange in the presence of this Pd catalyst selectively to the (E)-trisubstituted allylic acetates 2. 

Stereoselective allylic rearrangement. Irradiation of either chanoclavine-I (1) or isochanoclavine-1 (2) (ergot alkaloids) in the presence of sulfuric acid... 

Scheme 9 Phosphonium salt induced stereoselective allylic rearrangement.

Several papers are concerned with the threo-erythro stereoselectivity of the reaction of allylic organozinc reagents with carbonyl compounds. The addition (involving allylic rearrangement) of crotylzinc derivatives to various aldehydes occurs stereoselectively, and the relative amount of tAreo-alcohol increases with increasing steric demand of the group R tert-Bu, S4% f-Pr, 707o n-Bt, 46%) and in the sequence of metals Mg < Zn < Cd 3, 7). The temperature or the polarity of the solvent... 

In contrast to the few examples17-19 for the stereoselective sigmatropic rearrangements of acyclic allylic selenoxides, many examples9 16 20-29 have been reported for cycloalkenyl se-lenides. Table 8 shows examples demonstrating the pronounced selectivity for the anticipated suprafacial course of the rearrangement of cycloalkenyl selenoxides. 

Fully stereoselective allylic xanthate to allylic dithiocarbonate rearrangement was first observed with diastereomeric 2,3-unsaturated sugar derivatives 1 and 3 which on heating (140 UC for 1 to 2 h) gave the diastereomers of 3,4-unsaturated sugar dithiocarbonates 2 and 4, respectively43. 

A crucial step in the synthesis of thiotetronic acids involves a fully stereoselective allylic xanthate to dithiocarbonate rearrangement45. Rearrangement of ethyl ( )-(4S)-2-methyl-4-[methylthio-(thiocarbonyl)thio]-2-pentenoate (7) gives ethyl (/f)-(2.S)-2-methyl-2-[methylthio(curbonyl)thio]-3-pentenoate (8) as a single diastereomer with an -configurated double bond and at least 98% ee. This value was determined by Mosher derivatization of the alcohol prepared from the dithiocarbonate in three steps. The remainder of the (5S)-thiotetronic acid synthesis is shown in the scheme. 

An efficient method for the preparation of mono-heterocyclic compounds such as substituted tetrahydrofurans with high stereoselectivity via intramolecular SN2 O-cyclization of alkoxides has been reported. This is a nucleophilic displacement with a simultaneous allylic rearrangement (Scheme 321).1114... 

On the other hand, when tropone (41) is allowed to react with the phosphaalkynes 9a, 9b, or 9 e at a somewhat raised temperature in the molar ratio 1 1 in toluene, the reaction sequence ends with the formation of the diphosphapentacyclic compounds 45a-c [45]. In other words, this means that the initially formed 43 must have undergone a stereoselective, [8 + 2]-cycloaddition with a second equivalent of tropone (41). The hexacyclic species 44 thus formed experience a cyclopropyl-allyl rearrangement with subsequent [1,5]-H shift and ring inversion, finally to furnish the isolated pentacyclic compounds 45 [45]. The semiempirically... 

The transformation of an MBH adduct into stereoselectively allyl phos-phonates was initially reported by Morita et al. and then Janecki and Bod-alski reported that MBH adducts 11 and 12 can react with dialkyl chlorophosphites in the presence of triethylamine to give the corresponding phosphonates 13, which underwent the Arbuzov rearrangement upon heating to produce 2-methoxycarbonylallylphosphonates 14 with high stereoselectivity (Z E =95 5) and 2-cyanoallylphosphonates 15 with moderate stereoselectivity (E Z= 60 40 75 25) in moderate to high yields (Scheme 3.5). 

Stereoselective allylation can be accomplished using in situ aza-Cope rearrangement. The bicyclic compound 35, which is a precursor of Madangamine A, was treated with... 

The activation of non-activated C-H bonds is an important research field 14), In most cases, transition metal complexes have been used for this purpose 14-19). In this chapter, we wish to describe a stereoselective allylic C-H activation involving the thermal rearrangement of organoboranes (Scheme 7) 20-22). The observed stereochemistry may be best explained by a dehydroboration-rehydroboration mechanism, but mechanistic studies indicate that a more complex pathway involving a second molecule of BH3 may be involved. 

The first total synthesis of the intricate Stemona alkaloid (+ /—)-isostemofoline (224) was reported by Kende and coworkers 81) starting from 1,2-hexanediol (225) which was straightforwardly converted to 227 (Scheme 22) 82). Reductive cycUzation with sodium hydrosulfite in refluxing aqueous ethanol, and protection of the unstable pyrrole as tert-butyl carbamate, afforded 228 in five steps with 12% overall )deld. The key bicyclic ketone 231 was assembled by [4 + 3] cycloaddition of pyrrole 228 and diazoester 229 promoted by rhodium octanoate dimer, followed by enol silane deprotection, exo-specific hydrogenation, and nucleophilic decarboxylation (47% overall yield). Sodium methoxide-catalyzed aldol condensation of ketone 231 and furfural provided the Q-j/i-unsaturated ketone 232 whose olefin configuration was established by nOe studies. Allylation of 232 provided a 2.4 1 mixture of ketone 234 and the corresponding allylic enol ether 233, which could be converted to the former via a stereoselective Claisen rearrangement. 

---