Oxidative Deprotection of Benzyl Protecting Groups for Alcohols by an Electronically Tuned Nitroxyl-Radical Catalyst

The oxidative deprotection of benzyl (Bn) groups using nitroxyl-radical catalyst 1 and co-oxidant phenyl iodonium bis(trifluoroacetate) (PIFA) is reported. This catalyst is highly active for the oxidation of benzylic ethers because of the electronic tuning on account of the electron-withdrawing ester groups next to the catalytically active center. This catalytic system promotes deprotections at ambient temperature and has a broad substrate scope, including substrates possessing hydrogenation-sensitive functional groups, while the deprotection hardly proceeds when using well-known nitroxyl-radical catalysts such as 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO). The 1/PIFA system also promotes the deprotection of several benzylic protecting groups, including 2-naphthylmethyl (NAP) and 4-methylbenzyl (MBn) groups. Catalyst 1 was also effective for the direct synthesis of ketones and aldehydes from Bn ethers via deprotected alcohols using an excess of the co-oxidant PIFA.

Halogenation Reactions of Alkyl Alcohols Employing Methyl Grignard Reagents

Grignard reagents are commonly used as carbanion equivalents. Herein, we report an example of Grignard reagents acting as halide nucleophiles to form alkyl iodides and bromides. We establish that Grignard reagents can convert alkyl mesylates into alkyl halides, as well as be employed in a one-pot halogenation reaction starting from alcohols, which proceed through mesylate intermediates. The halogenation reaction is confirmed to occur by an SN2 pathway with inversion of configuration and is demonstrated to be efficient on multi-gram scale.

Recent approaches towards one-carbon homologation-functionalization of aldehydes

One-carbon homologation-functionalization in organic synthesis is a quite challenging and difficult task in terms of atom economy, ease of reaction, selectivity and number of steps involved. Due to the reactivity associated with most classes of carbonyls, these groups have always attracted a great deal of attention from synthetic chemists to transform them into various functionalities. In this context various researchers developed new methods for one-carbon extension-functionalization of carbonyls that serve as effective synthetic methodologies and are widely used in target-oriented and natural product synthesis. On account of the vast applicability associated with these transformations, herein we seek to summarize and highlight the important synthetic achievements in this advancing arena for various one-carbon homologation cum functionalization reactions of aldehydes and deep dive into some modern approaches adopted by organic chemists.

Cyanide-Free One-Pot Synthesis of Methacrylic Esters from Acetone

Methacrylic esters, represented by methyl methacrylate (MMA), are widely used as commodity chemicals. Here, the one-pot synthesis of methacrylic esters from acetone, a haloform and alcohols in the presence of an organic base is described. Using DBU as the organic base for the reaction of acetone, chloroform and methanol in acetonitrile afforded MMA in 66 % yield. When the solvent was replaced by benzonitrile, the product MMA was successfully purified by distillation. Applicability of this process to various alcohols was also investigated to show ethyl, phenyl, CF₃ CH₂ , and n-C₆ F₁₃ CH₂ CH₂ esters were obtained in moderate yields. The use of bromoform instead of chloroform resulted in the improvement of the yield, for example, methyl and n-C₆ F₁₃ CH₂ CH₂ esters up to 81 and 70 %, respectively. The reaction with deuterated starting materials acetone-d₆ and MeOH-d₄ , with DBU in acetonitrile afforded deuterated MMA (MMA-d₈ ) in 70 % yield.

One and Two-Carbon Homologation of Primary and Secondary Alcohols to Corresponding Carboxylic Esters Using β-Carbonyl BT Sulfones as a Common Intermediate

Herein we report the efficient one- and two-carbon homologation of 1° and 2° alcohols to their corresponding homologated esters via the Mitsunobu reaction using β-carbonyl benzothiazole (BT) sulfone intermediates. The one-carbon homologation approach uses standard Mitsunobu C-S bond formation, oxidation and subsequent alkylation, while the two-carbon homologation uses a less common C-C bond forming Mitsunobu reaction. In this latter case, the use of β-BT sulfone bearing esters lowers the p K_a sufficiently enough for the substrate to be used as a carbon-based nucleophile and deliver the homologated β-BT sulfone ester, and this superfluous sulfone group can then be cleaved. In this paper we describe several methods for the effective desulfonylation of BT sulfones and have developed methodology for one-pot alkylation-desulfonylation sequences. As such, overall, a one-carbon homologation sequence can be achieved in a two-pot (four step) procedure and the two-carbon homologation in a two-pot (three step) procedure (three-pot; four step when C-acid synthesis is included). This methodology has been applied to a wide variety of functionality (esters, silyl ethers, benzyls, heteroaryls, ketones, olefins and alkynes) and are all tolerated well providing good to very good overall yields. The power of our method was demonstrated in site-selective ingenol C20 allylic alcohol two-carbon homologation.

One-Carbon Homologation of Primary Alcohols to Carboxylic Acids, Esters, and Amides via Mitsunobu Reactions with MAC Reagents

A method is reported for the one-carbon homologation of an alcohol to the extended carboxylic acid, ester, or amide. The process involves the Mitsunobu reaction with an alkoxymalononitrile, followed by unmasking in the presence of a suitable nucleophile. The homologation and unmasking can even be performed in a one-pot process in high yield.