Hydroboration. 57. Hydroboration with 9-borabicyclo[3.3.1]nonane of alkenes containing representative functional groups

Abstract

The hydroboration of alkenes containing representative functional groups was examined with 9-borabicyclo[3.3.1]nonane (9-BBN) in order to extend the hydroboration reaction for the preparation of functionally substituted organoboranes. Terminal alkenes containing a remote functional group are hydroborated with a remarkable regioselectivity (>98% terminal), producing the corresponding stable organoboranes. 9-BBN hydroborates the allylic derivatives so as to place boron essentially on the terminal carbon atom (>97%). The directive effect is further enhanced (>99%) in the case of jS-methylallyl derivatives. The hydroboration of crotyl derivatives attaches boron predominantly at the 2-position, followed by an elimination-rehydroboration sequence.However, crotyl alcohol can be protected against elimination as the tert-butyl or tetrahydropyranyl ethers. The hydroboration-oxidation of ethyl crotonate involves a series of elimination, hydroboration, and condensation processes. In the vinyl, crotyl, and isobutenyl systems, the mesomeric effect of the substituent favors the placement of boron at the ^-position, while the inductive effect favors the «-position, with the former effect predominating in most cases. Acyclic ß-substituted organoboranes undergo rapid elimination. Nonpolar solvents and lower reaction temperatures decrease the rate of elimination. However, those derived from cyclic vinyl derivatives are relatively stable under neutral conditions, undergoing facile elimination in the presence of a base.

Regio- and Enantioselective N-Allylations of Imidazole, Benzimidazole, and Purine Heterocycles Catalyzed by Single-Component Metallacyclic Iridium Complexes

“…Standard procedures for hydroboration and oxidation gave complex product mixtures from reactions of bis-BOC-protected adenine derivatives 21j and 21l 77,78. However, hydroboration of the monomethoxytrityl(MMTr)-protected adenine derivatives 26a and 26b , followed by oxidation of the resulting alkylboranes in the presence of basic hydrogen peroxide gave MMTr-protected ( R )-3-(6-amino-9 H -purin-9-yl)-3-phenylpropan-1-ol) 27a and MMTr-protected ( S )-3-(6-amino-9 H -purin-9-yl)hexan-1-ol 27b in 82% and 77% yield (Equation 2).…”

Convenient Synthesis of a Polyester‐co‐Polyether Block for Assembling Biocompatible Hyperbranched Macromolecules

“…The second “block by block” approach includes the preliminary transformation of allyls of 9 into hydroxy–propyl groups by using the reaction with 9‐bora­bicyclo[3.3.1]nonane (9‐BBN) followed by oxidation of boroderivatives with hydrogen peroxide in the presence of sodium acetate. The use of these conditions prevents the degradation of fragile ester bonds . Dendron 11 was then synthesized by the grafting of three succinic blocks 12 followed by the final removal of the trityle protective group (Scheme ).…”

“…The use of these conditions prevents the degradation of fragile ester bonds. [ 23 ] Dendron 11 was then synthesized by the grafting of three succinic blocks 12 followed by the fi nal removal of the trityle protective group (Scheme 3 ). Using this pathway, the overall yield of 11 was 42% that is much higher than the yield of 10 observed with the step-by-step approach.…”

Design and Synthesis of a Cyclopentene Scaffold Mimicking Oseltamivir as a Novel Neuraminidase Inhibitor

“…The reaction was monitored by TLC and after the reaction completion (3 h) it was quenched with water (0.10 mL), evaporated to dryness, purified by silica gel column chromatography (25% EtOAc:hexanes), and crystallized in EtOAc:hexanes (1:1) to obtain 11 (0.41 g, 0.76 mmol, 68%) as white crystals. (4R,5R)-5,6-Dihydroxy-4-acetamido-3-benzoyloxymethyl-1-cyclopentene (12). TBAF (1 M solution in THF, 2.90 mL, 2.90 mmol) was added to a solution of 11 (1.45 g, 2.28 mmol) in THF (25 mL) at room temperature.…”