α‐Alkylierung von β‐Hydroxycarbonsäuren über 1,3‐Dioxan‐4‐on‐Enolate

Abstract

Highly Diastereoselective a -Alkylation of /I-Hydroxycarboxylic Acids Through Lithium Enolates of 1,3-Dioxan-4-onesFrom serine, B-hydroxyisobutyric acid ('Roche' acid) and B-hydroxybutync acid, the dioxanones 1 4 were prepared. The generation of the enolates of type I with LDA at -75" and alkylation gave products with trans-configuration whereas protonation of the 5-methyl-substituted enolate allowed access to the cis-configurated @-hydroxybutyric-acid derivative 12. Hydrolysis gave the free /I-hydroxy acids of 'syn'-and 'anti'-configuration. Alkylation of the 6-unsubstituted dioxanones 1 and 3 yielded predominantly products resulting from attack in the cis-position of the t-Bu group. The 'reactive' conformation of the enolates involved is tentatively derived from the product configuration. The selectivity of the alkylation is also discussed in terms of the results of an ah-initio calculation on the enolates M-P. A) Einleitung und

Synthesis and Properties of Monodisperse Chiral Dendrimers (up to Fourth Generation) with doubly branched building blocks: An intriguing solvent effect

“…All chiral building blocks, e.g., core triol 2 [15-171 and branch triol 3 [1][14][18], were prepared from the dioxanone 1 [19][20], which is easily accessible from the biopolymer P(3-HB)4) by aldol addition and reduction.…”

ChemInform Abstract: Mono‐ and Dialkylation of Derivatives of (1R,2S)‐2‐Hydroxycyclopentanecarboxylic Acid and ‐cyclohexanecarboxylic Acid via Bicyclic Dioxanones: Selective Generation of Three Contiguous Stereogenic Centers on a Cyclohexane Ring.

“…It is interesting to compare the bicyclic enolates described here with those of monocyclic dioxanone derivatives [9] [lo], see T and U. With Me group(s) at C(5) and/or C(6), the preferred trajectory of electrophile approach is always with relative topicity like (Si in T, antiperiplanar to the Me group).…”

“…After slow warming to room temperature and aqueous workup, the products 5a-j and 6a,b,f,g (Scheme 3 and Table 1 ) were obtained in 40-90% yields and, in the case of the cyclopentane derivatives 3a,b and of the formaldehyde acetal4a, with excellent diastereoselectivity. The much higher reactivity of the enolates J containing the five-membered ring allowed for alkylations with less reactive halides") than in the case of enolates K. Both bicyclic enolates were thermally much more stable") than the monocyclic analogues [9].…”

Asymmetric Electrophilic α‐Amination of Carbonyl Groups

“…Chiral 3‐hydroxy esters and 1,3‐dioxa‐4‐ones are well‐known substrates for the diastereoselective α‐alkylation reactions developed by Frater42 and Seebach43. These chiral compounds are available in both enantiomeric forms and have been aminated at the α‐carbon center with high to excellent stereoselectivity: 60−80% for 3‐hydroxy esters and > 95% for 1,3‐dioxa‐4‐ones.…”

“…D ‐Allothreonine43 and ( S )‐trifluorothreonine methyl ester45 have been synthesized diastereoselectively from the corresponding 3‐hydroxybutanoic and 4,4,4‐trifluoro‐3‐hydroxybutanoic acids protected as 1,3‐dioxa‐4‐ones.…”