(Z)-α-Boryl-crotylboron reagents via Z-selective alkene isomerization and application to stereoselective syntheses of (E)-δ-boryl-syn-homoallylic alcohols

Abstract

Stereoselective synthesis of (Z)-α-boryl-crotylboronate is developed.

Stereochemical Control via Chirality Pairing: Stereodivergent Syntheses of Enantioenriched Homoallylic Alcohols

“…Therefore, we surmise that aldehyde addition with E ‐reagent 4 might proceed via transition state TS‐6 to give E ‐isomer 6 a when employing acid ( R )‐ A as the catalyst (See the computational studies for more discussions). However, the main concern of this hypothesis is that stereoselectivity of the reaction might be moderate, because pseudo equatorial placement of the α‐CH 2 Bpin group of reagent 4 will develop steric interactions with chiral acid catalyst ( R )‐ A in the reaction transition state, as shown in our attempted asymmetric allylation studies with α‐Bpin‐substituted Z‐crotylboronate [26] …”

Stereochemical Control via Chirality Pairing: Stereodivergent Syntheses of Enantioenriched Homoallylic Alcohols

“…However,t he main concern of this hypothesis is that stereoselectivity of the reaction might be moderate,because pseudo equatorial placement of the a-CH 2 Bpin group of reagent 4 will develop steric interactions with chiral acid catalyst (R)-A in the reaction transition state, as shown in our attempted asymmetric allylation studies with a-Bpin-substituted Z-crotylboronate. [26] To test our hypothesis,a ldehyde allylation studies with boron reagent 4 were conducted using phosphoric acid (R)-A as the catalyst. As shown in Scheme 3, benzaldehyde was treated with boronate 4 at À45 8 8Ci nt oluene in the presence of 5mol %o facid( R)-A.G ratifyingly,a15:1 selectivity was observed with E-isomer 6a as the major component of the mixture after oxidative workup,a nd diol 6a was obtained in 89 %y ield with excellent enantioselectivity (> 99 % ee).…”

Hydroelementation of diynes

“…1,2,4,7,10,12,48 Hydroboration of conjugated and separated diynes, because of the increased complexity of their structure, is much more challenging in the case of selectivity control. Moreover, the possibility for carrying out mono-, bishydroboration, polyaddition reactions and cyclisation reactions with these reagents creates the possibility to obtain various products, which have been used in the synthesis of natural compounds, pharmaceuticals (e.g., anticancer rizoxin D, cytotoxic nannocystin Ax, ivorenolides), [111][112][113][114][115][116][117][118][119] dyes, 120 p-conjugated compounds, or heterocycles. [121][122][123] The information in this section is divided according to the type of reagent used: conjugated or separated diynes, as well as the formation of different products: enynes, dienes, heterocyclic compounds, and polymers.…”