[2 + 2] Cycloaddition reactions between ketenes, bearing amino‐, oxy‐, or halo‐ groups, and imines are recognized as being amongst the most important and direct routes to β‐lactams. Alkyl‐substituted ketenes also furnished the corresponding β‐lactams upon reaction with activated imines (iminoesters). In general, ketenes are generated from the appropriate acid chloride and a tertiary amine. The major or sole product of the cycloaddition is usually the cis‐β‐lactam, although a few exceptions showing trans selectivity are known. In this way β‐lactams with a widely varying substitution pattern at the C‐3 and C‐4 positions of the ring are constructed stereoselectively. The diastereoselection of the cycloaddition process can be controlled with variable success from chiral groups attached to either the ketene or the imine component, or alternatively to both. This method, in turn, has proved to be valuable for the synthesis of precursors of important β‐lactam antibiotics, and new successful applications can be expected in the near future.
In the last two decades, the better understanding of the mechanistic aspects of the beta-lactams' biological activity and their inhibition, and the chemical exploitation of beta-lactams as synthetic intermediates in organic chemistry, have experienced a continuous and somewhat complementary advance. A prerequisite for such a development has been the accessibility of enantiopure beta-lactams. The latter are now routinely prepared most often through the ketene-imine cycloaddition reaction, also termed the Staudinger reaction. This review accounts for the recent progress made in the asymmetric synthesis of beta-lactams (with special emphasis in the Staudinger reaction approach), as well as in their use as synthetic intermediates en route to natural products, including alpha- and beta-amino acids and peptides derived therefrom.
Spectroscopic (Raman) and gravimetric (acid titration, TGA) techniques were compared to determine the functionalization degree of multiwalled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) with poly(L-lactic acid) (PLLA), following a two-step oxidation/esterification process. After oxidation with HNO 3 , carbon nanotubes (CNTs) were activated with thionyl chloride to the corresponding acid chlorides and then grafted with PLLA. FTIR spectroscopy confirmed the formation of very similarly grafted CNT-PLLA materials for both SWNT and MWNT. But, according to chemical titration and TGA results, oxidized and esterified SWNT showed a significantly higher degree of functionalization than their MWNT counterparts. Contrary to these observations, the increase of the Raman I D /I G ratio was higher for MWNT-derived materials than for the SWNT counterparts. Therefore, it was concluded that this spectroscopic technique is unsuitable for the quantitative determination of the degree of functionalization when MWNT and SWNT are compared.
The synthetic utility of hydrosilanes under the influence of trimethylsilyl-based reagents as new reducing systems is described. 1,1,3,3-Tetramethyldisiloxane (TMDS) reagent in combination with iodotrimethylsilane or bromotrimethylsilane produces alkyl halides from aldehydes in good to excellent yields. Polymethylhydrosiloxane (PMS) in the presence of iodotrimethylsilane also produces benzyl iodides in excellent yields. On the contrary, PMS reagent was found unsuitable for the synthesis of benzyl bromides. Similarly, TMDS reagent in combination with trimethylsilyl triflate produces symmetrical ethers from aldehydes without concomitant formation of competitive products. Under similar conditions, PMS reagent failed to provide the expected symmetrical ethers and Friedel–Crafts products were formed. Reduction of quinones to hydroquinones is also described.
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