Stereoselective Synthesis of Alcohols, XX. Diastereoselective Addition of γ‐Alkoxyallylboronates to Aldehydes

Abstract

Both the (Z)-and the (E)-y-alkoxy-substituted allylboronates 1 and 3 have been prepared. They add to aldehydes with a diastereoselectivity generally exceeding 90% to give the syn-(2) and anti-diol derivatives 4, respectively. The structure of one of these adducts has been established by conversion into exo-brevicomin (26). Stereoselektive Synthese von Alkoholen, XX I). -Diastereoselektive Addition von y-Alkoxyallylboronsaiureestern an AldehydeSowohl (Z)-als auch (E)-y-Alkoxyallylboronsaureester 1 und 3 wurden dargestellt. Sie addieren sich an Aldehyde zu den syn-(2) bzw. anti-Diol-Derivaten 4, wobei die Diastereoselektivitaten meist 90% iibersteigen. Die Struktur eines der Addukte wurde durch Umwandlung in exo-Brevicomin (26) gesichert.In the synthesis of compounds having multiple adjacent stereocenters it is desirable to attain high diastereoselectivity in the steps that create the individual stereocenters. To achieve this in molecules with acyclic skeletons new methods are currently being developed'). Among the strategies to achieve this goal it would be optimal to create these stereocenters with the proper relative configuration concomitant with the construction of the molecular skeleton. This could be realized, e. g., by stereocontrolled aldol additions 3, or aldol-analogous reactions4). In this paper we would like to focus on a stereocontrolled generation of a 1,2-diol unit by addition of y-alkoxyallylboronates 1 and 3 to aldehydes. This reaction has been studied similarly by Wuts') independent of our own investigations6). Related studies involving other y-oxygenated allylmetal compounds have been reported 79839).According to our previous experience with the diastereoselective carbon -carbon bond formation using allylboronates'oJ') we expected that the (Z)-y-alkoxyallylboronates 1 should lead to the syn-1,Zdiols 2 on addition to aldehydes, whereas the (E)-counterpart 3 should give the anti-1,2-diols 4 with very little stereocrossover.

Stereodivergent Syntheses of Highly Substituted Enantiopure 4‐Alkoxy‐3,6‐dihydro‐2H‐1,2‐oxazines by Addition of Lithiated Alkoxyallenes to Carbohydrate‐Derived Aldonitrones

“…The viability of the use of different alkoxy groups at the allene moiety was studied, because the benzyloxy and the (trimethylsilyl)-ethoxy substituents allow transformations and cleavage conditions not possible with simple alkoxy groups such as methoxy groups. The known literature synthesis of (trimethylsilyl)ethoxyallene [15] was simplified by performing the two steps-propargylation of 2-(trimethylsilyl)ethanol and subsequent base-promoted alkyne-allene isomerization-as a one-pot procedure [Equation (1)]. (1)…”

“…[15] : 2-(Trimethylsilyl)ethanol (4.12 g, 34.9 mmol) was slowly added at 0°C to a suspension of sodium hydride (1.40 g, 35.0 mmol, 60 % in paraffin oil) in tetrahydrofuran (17 mL). After stirring for 2 h at room temperature, the suspension was cooled to 0°C, and propargyl bromide (4.57 g, 38.9 mmol) was added dropwise.…”

Synthesis of 5‐Acetyloxazoles and 1,2‐Diketones from β‐Alkoxy‐β‐ketoenamides and Their Subsequent Transformations

“…The method allows the preparation of oxazoles with alkyl and aryl substituents at C-2 and C-4 in yields up to 98 %. With respect to R 3 , the substitution pattern was then extended to hetaryl, alkynyl, and acetyl groups (Table 2, entries 2, 5, and [13][14][15][16]. It should also be noted that only a few alternative synthetic methods are available for the direct introduction of perfluoroalkyl groups at C-2 of an oxazole derivative (Table 2, entries 3, 6, 10, and 12).…”

“…The required gphenyl-substituted 2-(trimethylsilyl)ethoxyalkyne 16 was synthesized by palladium-catalyzed coupling of iodobenzene with (2-trimethylsilyl)ethyl propargyl ether. According to the standard conditions for the enamide synthesis, the alkyne was isomerized in situ to an a-lithiated alkoxy-A C H T U N G T R E N N U N G allene, [14] and subsequent treatment with pivalonitrile and TFA generated enamide 17 in moderate yield. Cyclization mediated by TFA led to the formation of 2-phenylacetylsubstituted oxazole 18 (59 %) as the sole product.…”

Recent Developments of Boryl Anions: Boron Analogues of Carbanion

“…In the case of 54, one can expect that the lithium atom may act as a Lewis acid to achieve the conjugate addition. 132 In contrast, the reaction of boryllithium 52 with 2-cyclohexen-1-one led only protonation of 52 (as judged by 1 [141][142][143][144][145] It should be noted that transition-metal-catalyzed silaboration of an ,-unsaturated ketone affords complementary products, -boroxyallylsilane.…”