Reactivity of (R)‐2‐tert‐Butyldihydrooxazole Derivatives Prepared from Serine and Threonine: Novel, Versatile Chiral Building Blocks

Abstract

We have therefore investigated the reaction of the lithium salt 1 with chlorotrimethylsilane and chlorotriphenylsilane.["' After workup, we obtained the diazo derivatives 7b (90% yield) and 8b (82% yield), respectively. However, when the reactions were monitored by spectroscopy, the N-silylated nitrile imines 7a and 8a were characterized at -40°C. Compared with the spectroscopic data for the diazo derivatives (2b, 3b, 4b, 7b, 8b), nitrile imines (2a, 7a, 8a) show a 3'P-NMR chemical shift at high field and an extremely broad IR absorption (see Table 1). 7a is only stable in solution below 0"C, but 8a is still observable after a few hours, at room temperature. The nitrile imines 7a and 8a react with methylacrylate to give the expected [ 2 + 3 ] adducts 9 and 10, respectively, in good yield.['o~l'J With methanol the diazo compound 4b is obtained in 92% yield along with quantitative amounts of methoxytrimethyl-and methoxytriphenylsilane. This last reaction is analogous to that observed upon filtration of 2a over silica gel, and probably involves the primary formation of nitrile imine 4a. S S RiSiCI I 1 0 0 RT 11 1 M R2P-CrN-N-SiR; + R2P-C-SiR; -40°C I 1 70.80 I Nz 7b,8b C02Me Y 9.10 7, 9: R = iPr2N. R' = M e 8.10: R = iPr2N, R' = P hThese results as a whole strongly suggest that, in contrast to what is generally admitted, nitrile imines (a series) are the kinetic products of the electrophilic attack of diazolithium salts such as 1, whereas the isomeric diazo derivatives (b series) are the thermodynamic products. The stability of nitrile imines is strongly dependent on the steric hindrance of the nitrogen substituent.

Selective Carbon–Carbon Bond Cleavage of Cyclopropylamine Derivatives

“…This iminium ion can be trapped by an exogenous nucleophile to provide vinyl halide products of type 103 . Alternatively, in pathway b initial elimination of HHal sets up a strained aminocyclopropene which can undergo rearrangement to vinyl carbene 104 and subsequent reaction. − In this sequence, bond b undergoes cleavage.…”

Enantioselective Cyclopropanation of Allylic Alcohols. The Effect of Zinc Iodide

“…The obvious importance of enantiopure cyclopropanes as end products or synthetic intermediates led to the development of enantioselective versions of this reaction. There are numerous auxiliary-based methods such as the tartrate-derived acetal 1 pioneered by Yamamoto that illustrate the high levels of diastereoselectivity that can be obtained (Figure ) . The use of stoichiometric quantities of chiral additives to modify the reagent has also met with considerable success .…”

Tetrazolic Acid Functionalized Dihydroindol: Rational Design of a Highly Selective Cyclopropanation Organocatalyst

“…Early development in stereospecific cyclopropanation utilized a Simmons-Smith type of process, in which iodomethylzinc reagents generated by different methods allowed the enantioselective cyclopropanation of structurally diverse cyclic and acyclic alkene systems to be carried out by utilizing the chiral pool, chiral auxiliaries, and catalytic asymmetric cyclopropanation, using iodomethylzinc with C 2 -symmetric disulfonamide ligands and Ti-taddolates …”