Glycosylidene Carbenes, Part 29 , Insertion into B−C and Al−C Bonds: Glycosylborinates, -boranes, and -alanes

Abstract

Insertion of the glycosylidene carbenes derived from the diazirines 1, 14, and 15 into the BÀalkyl bond of the B-alkyl-9-oxa-10-borabicyclo[3.3.2]decanes 5, 6, and 7 yielded the stable glycosylborinates 8/9 (55%, 55 : 45), 10/11 (31%, 65 : 35), 12/13 (47%, 60 : 40), 16/17 (55%, 55 : 45), 18/19 (47%, 45 : 55), and 20/21 (31%, 30 : 70). Crystal-structure analysis of 17 and NOEs of 9 and 19 show that 17, 9, and 19 adopt similar conformations. The glycosylborinates are stable under acidic, basic and thermal conditions. The unprotected glycosylborinate 25 was obtained in 80% by hydrogenolysis of 12. Insertion of the glycosylidene carbene derived from the diazirine 1 into a BÀC bond of BEt 3 , BBu 3 , and BPh 3 led to unstable glycosylboranes that were oxidised to yield the hemiacetals 29 (55%), 31 (45%), and 33 (48%), respectively, besides the glucals 30 (13%), 32 (20%), and 34 (20%), respectively. Insertion of the glycosylidene carbenes derived from 14 and 15 into a BÀC bond of BEt 3 led exclusively to hemiacetals; only 15 yielding traces of the glucal 40 besides the hemiacetal 39. The glycosylidene carbene derived from 1 reacted with Al( i Bu) 3 and AlMe 3 to generate reactive glycosylalanes that were hydrolysed, yielding the C-glycosides 46 (21%) and 49 (30%), respectively, besides the glucals 48 (26%) and 51 (30%); deuteriolysis instead of protonolysis led to the monodeuterio analogues of 46 and 49, respectively, which possess an equatorial 2 H-atom at the anomeric center.Introduction. ± Glycosylidene carbenes, generated by thermolysis or photolysis of diazirines, insert into XÀH bonds to form O-, C-, and N-glycosides [2 ± 6], glycosylphosphines [7], and glycosylstannanes [8].We considered that the nucleophilic attack 2 ) of glycosylidene carbenes (e.g., 2) on triligated B or Al derivatives should lead to tetraligated intermediates, such as the Bylide 3 (Scheme 1). These ylides are expected to undergo (axial or equatorial) migration of a B or Al substituent, respectively, as shown in Scheme 1 for glycosylboranes 4, by analogy to the known migration of a B substituent in tetraligated borates that possess a vicinal leaving group [12] [13]. They should lead to C(1)-substituted

Boranes in Organic Chemistry 1. α-Carbonylalkyl- and β-Oxyalkylboranes in Organic Synthesis

“…Thus, 3-methyl-2-cyclohexenone 128 formed 70% of diol via β-oxyalkylborane 129 (Scheme 57) [79][80][81][82]. Piperitone 130 formed a mixture of trans-diequatorial diols in equal quantities also via β-oxyalkylborane 131 [72]. The same reaction was found for 5-phenyl-2-cyclohexenone 132 which was transformed to the corresponding diols via β-boryl alkyl ether 133 [81].…”

“…Vasella et al [71][72][73] showed that the insertion of glycosylidene carbenes into a boron-carbon bond of BEt 3 led to unstable glycosyl boranes, while insertion into a boron-carbon bond of boronic esters yielded stable anomeric glycosyl borinates. The glycosylidene carbenes were generated by thermolysis or photolysis of glycosylidene diazirines.…”

β-Glycosyl Trifluoroborates as Precursors for Direct α-C-Glycosylation: Synthesis of 2-Deoxy-α-C-glycosides

“…Therefore, the Suzuki–Miyaura coupling reaction was employed to construct C-arylated (or vinylated) glucal derivatives 2 from C1-sp 2 pinacol boronate 6 (Figure B). − This provides access to 2-deoxy-β- C -glucosides . As for C1-sp 3 glycosyl boronates, surprisingly, simple C-1 borylated monosaccharides have not been reported, except for C-1 alkylated and B-substituted monosaccharides prepared by the B–C bond insertion reaction of a glycosyl diazirine. , We envisioned that potassium β-glycosyltrifluoroborates such as 7 , which are obtainable by hydrogenation of sp 2 -boronate such as 6 (Figure C), would undergo a Ni-catalyzed metallaphotoredox coupling reaction with aryl or vinyl halides. − Single-electron oxidation of 7 would generate glycosyl radical 8 , which is expected to adopt a standard chair conformation with α-radical orientation stabilized by the inner cyclic oxygen atom. − Then, stereoinvertive coupling should proceed to give 5α selectively (Figure S1). , This approach would be complementary to the method via compound 2 and might be useful to synthesize analogues of native 2-deoxy glycosides (usually β- O -glycosides).…”

“…13 As for C1-sp 3 glycosyl boronates, surprisingly, simple C-1 borylated monosaccharides have not been reported, except for C-1 alkylated and B-substituted monosaccharides prepared by the B−C bond insertion reaction of a glycosyl diazirine. 14,15 We envisioned that potassium β-glycosyltrifluoroborates such as 7, which are obtainable by hydrogenation of sp 2 -boronate such as 6 (Figure 1C), would undergo a Ni-catalyzed metallaphotoredox coupling reaction with aryl or vinyl halides. 16−18 Single-electron oxidation of 7 would generate glycosyl radical 8, 2 which is expected to adopt a standard chair conformation with α-radical orientation stabilized by the inner cyclic oxygen atom.…”

Glycosylidene Carbenes, Part 30, Crystal Structure of a Glucose-DerivedN,N-Unsubstituted Alkoxydiaziridine

“…and 57.9(3)8, respectively. Furthermore, the diaziridine ring does not pivot about C (1) in an attempt to bring one of the N-atoms closer to an ideal axial or equatorial position; rather the mid-point of the NÀN bond almost bisects these positions. The angles between the plane defined by O(5)ÀC(1)ÀC (2), and the C(1)ÀN(1) and C(1)ÀN(2) vectors are 33.0(4) and 32.0(3)8, respectively.…”