New silicon-phosphorus reagents in organic synthesis. Carbonyl and conjugate addition reactions of silicon phosphite esters and related systems

Synthesis of Cortistatins A, J, K and L

“…The latter substance was first prepared in its antipodal form by Danishefsky and coworkers 27 (from the Hajos−Parrish enedione) 28 in the context of their synthesis of taxol, and was also selected as a starting material by the Nicolaou-Chen 6 and Sorensen 18 groups in their research directed toward the synthesis of cortistatins. As illustrated in Figure 4, phosphoniosilylation 29 33 in dichloromethane at 45 ºC for 5 h led to highly efficient ring-closing metathesis, forming the tetracyclic diene 18 (>95% yield, 11-g scale). In our initial route design we had planned to hydrogenate selectively the disubstituted alkene within this product and then epoxidize the remaining (tetra-substituted) alkene; however, we were not able to achieve selective 1,2-reduction of the diene.…”

On the discovery and development of tandem 1,4- and 1,2-addition of phosphites to 1-azadienes

“…A possible explanation for this unexpected behavior of dialkyl trimethylsilyl phosphite could also be found in the reaction mechanism of dialkyl trimethylsilyl phosphite additions described in literature. 36 In this mechanism, coordination of the nitrogen lone pair with the silicon atom is suggested, which brings the (bulky) nucleophile in close proximity of the electrophilic center. Because of the presence of both a nucleophilic and an electrophilic center in the silylated phosphite, the subsequent transformation would then occur via a classical "push-pull" mechanism (Scheme 9) and is accelerated.…”

“…It was shown on several occasions in literature that this type of intramolecular phosphonium to phosphonate conversion is disfavoured for trialkyl phosphites. 36 In fact, when aliphatic aldehydes are heated in the presence of trialkyl phosphites, only up to 40% of the α-alkoxy phosphonate has been reported, [37][38] which most likely originates from an intermolecular alkyl transfer. 39 O-to N-alkyl transfer does not occur, and aminophosphonate formation relies on the presence of other dealkylation agents.…”

Niobium (V) Chloride Catalyzed Abramov Reaction: An Efficient Protocol for the Preparation of α-Hydroxy Phosphonates

“…at rt for 2 h, there was no evidence of the formation of silyl phosphite or any other reaction product on 1 H NMR. 6 Therefore, the nucleophile was not silyl phosphite, but trialkyl phosphite. 5b,5d In summary, we have developed a new protocol for the synthesis of α-hydroxy phosphonates using catalytic amount of NbCl 5 .…”

“…6 These methods often met some disadvantages of difficult conditions such as high reaction temperature, longer reaction time and dried conditions. Moreover, the yields were not always good and sometimes byproducts were obtained, and esters of α-hydroxy alkyl phosphonic acid were often cleaved to regenerate the starting carbonyl compounds when strong alkaline mediums were used.…”