An Exploratory Study of Silylated Amino Boronic Ester Chemistry

Synthesis of a (β-acetamido-α-acetoxyethyl)boronic ester via azido boronic esters

“…Unfortunately, hydrogenation under these acidic conditions was accompanied by deacetylation to the a-hydroxy boronic ester 10, which proved unstable on attempted purification. Prompt acetylation of 10 led to pinanediol 1-acetoxy-2-acetamidoethylboronate (11). When the hydrogenation was carried out in methanol, deboronation of 10 occurred and the product isolated on treatment with aqueous sodium tetraphenylborate was ethanolamine tetraphenylborate (12), which showed a characteristic pair of triplets in the 1 H NMR at d 2.88 and 3.66, J = 5.3 Hz, and matched an authentic sample of 12 prepared from aqueous ethanolamine and hydrochloric acid with sodium tetraphenylborate.…”

“…Alkoxides are the only common nucleophiles that fail to displace chloride from certain a-chloro boronic esters, including b-azido boronic esters, as noted above [11][12][13][14][15]. The exothermic character of reactions that replace B-C bonds by B-O bonds [24] is undoubtedly a major factor in some cases.…”

“…Alkoxide displacements fail when there is another pathway accessible for the decomposition of the (a-chloroalkyl)(trialkoxy)borate intermediate, b-elimination in the case of the azido compounds and perhaps dissociation to an a-chloroallylic anion in the case of certain a-chloro boronic esters [12]. The reason for failure of silylated b-amino boronic esters to undergo a-chloro displacement by alkoxide was not clear [11]. In retrospect, it appears all the more remarkable that b-elimination of boron and oxygen from (b-alkoxy-a-chloroalkyl)(trialkoxy)borates was not evident in the assembly of a sequence of four adjacent benzyloxy substituted carbons for an asymmetric synthesis of ribose [25], though it could have been an unrecognized factor in the inefficiency of the process for introducing a fifth carbon in the same manner.…”

“…The synthesis of silylated (2-amino-1-chloroethyl)boronic esters from silylated aminomethyl boronic esters has been described [11]. The 1-chloride was displaced by sodio(methanethiol), and desilylation and treatment with trimethylsilyl isocyanate led to the [2-ureido-1-(methylthio)ethyl]boronic ester [11]. It appeared that dimethylamino substitution was also successful, though the product was not completely purified.…”

“…It appeared that dimethylamino substitution was also successful, though the product was not completely purified. Attempts to displace the chloride with benzyl oxide failed [11]. Attempted insertions of (dichloromethyl)lithium into silylated a-benzylamino-or a-formamido-b-trityloxy boronic esters failed [11].…”