Allyltributyltin derivatives

Some 2,3-diazabicyclo[2.2.1]heptene derivatives, for example, 175, with an aryl iodide and allyltributyltin in the presence of [Pd(allyl)Cl]2 in toluene provide unexpected products 176. It is interesting to note that aryl iodide is recovered almost completely but no reaction can be observed in its absence. When the aryl iodide is replaced by a Lewis acid, good yields of 176 are obtained. The reaction is very slow in toluene, but in ionic liquid [bmim]PF6 the reaction rate is significantly enhanced (Equation 21) <2005SL2273>. 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

The Pd-catalyzed reaction of 211 with allyltributyltin to give 212, followed by N-alkylation to 213, afforded the 1-benzazepine derivative 214 in high yield on Ru-catalyzed ring-closing metathesis with Grubbs I catalyst 159 (Scheme 28) <2005JOC1545>. 

Asymmetric radical reactions. Curran et al,3 report several asymmetric reactions of radicals derived from Oppolzer s camphorsultam. Thus the reaction of the iodosultam 1 with allyltributyltin initiated by triethylborane provides an epimeric... 

More recently, Benaglia and coworkers reported that the allylation reaction of a-iminoesters proceeded to give the homoallyl alcohols with moderate enantioselectivities (Scheme 9.4).9 The chiral diimine that has a chiral 1,2-cyclohexyldiamine framework was used as the ligand. The reaction of a-iminoesters and allyltributyltin in the presence of AgOTf and diimine ligand proceeded to afford chiral amino acid derivatives with moderate enantioselectivities. 

A mixture of cyclopropylmethyl bromide derivative (0.4 mmol), allyltributyltin (0.8 mmol), and AIBN (5 mol%) in dry benzene (3 ml) was degassed by bubbling argon gas for 30 min. The mixture was refluxed for 12 h. After removal of the solvent, aq. KF solution (5 ml) and ether (10 ml) were added to the residue and stirred for one hour. After filtration and separation, the organic layer was dried over Na2S04. After removal of the solvent, the residue was chromatographed on silica gel using ethyl acetate/hexane to afford a diallyl product [95]. 

Isoprenylated Quinones.—Chemistry. An efficient method has been described for the preparation of ubiquinone-1 (221) and plastoquinone-1 (223) from the parent quinone and allyltributyltin. The synthesis of ubiquinone-10 by isoprenoid chain-elongation of a ubiquinone-1 derivative has been reported.The sul-phone derivative of the protected ubiquinol-1 (224) on reaction with solanesyl bromide (225) and McjCOK gave the sulphone (226) in 90% yield. Benseker reduction to remove the PhCH2- and PhS02-groups, followed by oxidation in air, afforded ubiquinone-10 (222). 

Enholm [12] has also prepared an enantiomerically pure soluble polymer support 82 by couphng xylose-derived chiral auxiliary 81 with 77 (Scheme 18). The chiral support was then treated with bromopropionic acid 83 to give substrate 84. Eree radical allyl transfer from allyltributyltin imder thermal conditions provided 85 in 93% yield, and basic cleavage from the resin gave (R)-(-)-2-methylpent-4-enoic acid 86 in 80% yield and 97% ee, with a 92% yield of recovered 82. Previous studies of the same process in solution had found the addition of Lewis acids to be crucial for high selectivities to be obtained. Interestingly, the addition of Lewis acids to the reaction on polymer support led to cleavage of the carbohydrate from the polymer backbone. En-... 

The use of chiral auxiliaries proved to be particularly efficient for the preparation of optically pure materials via radical reactions [29]. Yamamoto et al. [30] examined the free-radical reaction of an a-bromoglycine derivative 35 having a chiral auxiliary with allyltributyltin 30 to give allylated product 36 (Sch. 14). Although the use of 0.1 equiv. 

Bromo-acetal 151 was subjected to tributyltin hydride promoted radical cyclization and afforded a stereoselective 8-C-methyl derivative 152. (O Scheme 40) Similarly, treatment of 151 under Keck s conditions [62] with allyltributyltin and AIBN resulted in the formation of a bis-C,C-glycoside 153 [63]. 

In 1991, the syntheses of sialyl C-glycosides were introduced by Bednarski et ah, Paulsen et ah, and Vasella et al. Bednarski and Paulsen exploited a 2-halogeno derivative of Neu5Ac as a precursor of the anomeric radical, which was anticipated to be stabilized by the capto-dative effect of the C1 carboxylate group and ring oxygen. Bednarski reacted the 2-chloro derivative 195 with allyltributyltin and a catalytic amount of bis(tributyltin), and photolized for 18 h to obtain C-glycoside 196 as a 1/1 of anomeric mixture in 65% yield (O Scheme 53) [129]. 

To improve the diastereoselectivity of C-glycosidation, Paulsen s group exploited 2-bromo-3-hydroxy derivative 21. As a result, the reaction of 21 with allyltributyltin and AIBN at 60 °C provided 197 (69%), favoring a-glycoside in the ratio of 3/1 (O Scheme 54) [130]. 

Exposure of an allylstannane to halopyranoses in the presence of AIBN enables access to the corresponding allyl adducts [95] this procedure is useful for C-allyl glycosidation [96]. To enhance the practical utility of C-glycosidation, the effect of conformational restriction on a- and / -selectivity was studied in free-radical allylation at the anomeric positions of phenylselenoxylose derivatives of fixed conformation (Scheme 12.37) [97]. Treatment of a phenylthionocarbonate of 2 -deoxynucleo-side [98] and an iodopyrrolidine ]99] to allyltributyltin/AIBN resulted in substitution of the allyl moiety. 

The Pd-eatalysed reaction of 48 with allyltributyltin to give 49, followed by Af-alkylation to 50, then gave the 1-benzazepine derivative 51 in high yield on Ru-catalysed ring closing metathesis <05JOC1545>. 

An additional example illustrating the utility of thiophenyl glycosides in free radical mediated C-glycosidations was reported by Waglund, et a/.6 As shown in Scheme 5.2.4, 1-methylcarboxy-l-phenylthioglycosides were treated with allyltributyltin and irradiated with ultraviolet light. The resulting C-allyl derivatives were formed in approximately 30% yields with the stereochemical... 

The components used in these two sequences cannot be interchanged without some adjustment. Thus the Z-substituent in the radical 7.74 is necessary for an efficient reaction—in its absence the allylstannane has to be used in large excess. Similarly, a vinylogously X-substituted radical 7.79, derived from cyclohexenone 7.78, will have a high-energy SOMO it does not add at all to allyltributyltin 7.75, but it does add to the allylstannane 7.80 equipped with a Z-substituent to give the radical 7.81 and hence the ester 7.82.1026... 

Allyltributyltin (5) is the most commonly used reagent for carrying out allylation reactions via a free radical fragmentation process [5]. Keck reported the first practical use of allyltributyltin for free radical allylation reactions in 1982 in the context of a synthesis of perhydrohistrionicotoxin [6]. Heating bromide 4 with allyltributyltin in the presence of AIBN as a radical initiator gave the allylated derivative 6 (Scheme 3) in high yield with complete control of stereochemistry. Similar transformations had proven to be very difficult by standard ionic reactions. 

Several recent examples serve to illustrate the potential of allyltributyltin for organic synthesis (Scheme 4). Baldwin and Easton have independently shown that glycine derivatives undergo efficient allylation reactions, providing an avenue for the preparation of non-natural amino acids [8]. Subsequently, addition of glycine-derived radicals to allyltributyltin was shown by Hamon to occur with stereoselectivity if the radical bears an ester chiral auxiliary [9]. An alternative method for the preparation of amino acid derivatives via allylation of carbamoyloxy radicals that are incorporated into an oxazolidin-2-one ring system was reported by Kano... 

In Section 12.7.B, the Stille coupling reaction reacted a vinyl tin compound with a vinyl triflate, in the presence of palladium(O). Tetravalent tin complexes add to aldehydes and ketones, in the presence of a Lewis acid. Allyltin complexes are, by far, the most widely used of these compounds.297 a typical example is taken from the work of Keck, in which a chiral aldehyde (455) was treated with allyltributyltin, in the presence of various Lewis acids. S As shown in Table 12.19, a mixture of syn (456) and anti (457) products was obtained. The ratio of 456/457 was dependent on the structure of the R group in 455, the solvent and the Lewis acid.The anti product (457) was obtained by using the tert-butyldimethylsilyloxy derivative (sec. 7.3.A.i) of 455 with 2 equivalents of boron trifluoride in dichloromethane. The syn product is obtained preferentially when the benzyloxy derivative of 455 is used with titanium tetrachloride in dichloromethane.298... 

C-6-Allylated pyranoses and C-S-allylated fliranoses were prepared from the corresponding iodides by Keck radical coupling with allyltributyltin (e.g. 44->4S). The selective formation of product 47 in the chain-extension of the acetal-protected uronic acid derivative 46 by radical addition of methyl acrylate using Barton s method was ascribed to addition from the less hindered side to a conformationally stable radical intermediate the peracetate 48 furnished a 1 1 mixture of 49 and 50. ... 

Radical reactions have been further utilised in the construction of branched-chain sugars. Unsaturated iodide (24) with allyltributyltin or Bu3SnH and methyl acrylate has afforded the doubly-branched derivatives (25) and (26) by way of intra- and then inter-molecular radical processes. Similarly, bromide (27) gave the 3-C-allyl derivative (28) which, after transformation into enal (29), underwent a further radical cyclisation (Scheme... 

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