Facile reduction of alkylamino-, anilino-, and pyridyl-N-oxides can be achieved via the use of diboron reagents, predominantly bis(pinacolato)- and in some cases bis(catecholato)diboron [(pinB)2 and (catB)2, respectively]. Reductions occur upon simply mixing the amine N-oxide and the diboron reagent in a suitable solvent, at a suitable temperature. Extremely fast reductions of alkylamino- and anilino-N-oxides occur, whereas pyridyl-N-oxides undergo slower reduction. The reaction is tolerant of a variety of functionalities such as hydroxyl, thiol, and cyano groups, as well as halogens. Notably, a sensitive nucleoside N-oxide has also been reduced efficiently. The different rates with which alkylamino- and pyridyl-N-oxides are reduced has been used to perform stepwise reduction of the N,N’-dioxide of (S)-(–)-nicotine. Because it was observed that (pinB)2 was unaffected by the water of hydration in amine oxides, the feasibility of using water as solvent was evaluated. These reactions also proceeded exceptionally well, giving high product yields. In constrast to the reactions with (pinB)2, triethylborane reduced alkylamino-N-oxides, but pyridine N-oxide did not undergo efficient reduction even at elevated temperature. Finally, the mechanism of the reductive process by (pinB)2 has been probed by 1H and 11B NMR.

The dual role of the bicyclic amidine base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was demonstrated in a synthesis of terminal aryl- and styryl-acetylenes. Mechanistically, a tandem process involving elimination/Umpolung/protonation occurs in a single step to generate terminal aryl- and styryl-acetylenes from geminal dibromoalkenes. The key to the success of this transformation lies in the organobase-mediated generation of the acetylide from the 1-bromoalkynes at room temperature. The unique characteristics of DBU as an inherently safer reagent make it an attractive alternative to previous systems wherein required pyrophoric reagents and nonambient temperatures remain unsolved issues. The procedure does not work for the synthesis of alkyl-acetylenes.

Sugar–iminosugar hybrid molecules made up of D‐glucose and D‐galactose with pyrrolidine‐based iminosugars, viz. 1,4‐dideoxy‐1,4‐imino‐L‐xylitol and 1,4‐dideoxy‐1,4‐imino‐L‐lyxitol, are synthesized from glycal epoxides and found to be moderate glycosidase inhibitors. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Two well known synthetic organic reactions Ramirez olefination and Corey-fuchs reactions are integrated in one-pot sequential manner for the synthesis of arylacetylenes and 1,3-enynes starting directly from commercially available aldehydes. The bicyclic amidine 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) along with additive NaOH not only exclusively afforded the terminal alkynes directly from the aldehydes, but also enhanced the reaction rate. The dynamic nature of DBU also facilitated the isolation of 1-bromoalkynes intermediate products. Selection of additive from NaOH and H 2 O served as a switch for the synthesis of terminal alkyne and 1-bromoalkynes, respectively. Scheme 1. Schematic for the one-pot Synthesis of terminal alkynes and 1-bromoalkynes using DBU.

Two pyrrolidine-based imino sugars have been synthesized in an efficient manner, using regiospecific amination, ring closing metathesis, and diastereospecific dihydroxylations as key steps. These azasugars are found to be moderate inhibitors of glycosidases.

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The nucleophilic reactivity of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) was completely controlled by the formation of monohydrate (DBU⋅H2O) in the synthesis of 1‐(bromoethynyl)arenes from 1,1‐dibromoalkenes. Differential reactivity of DBU in protic solvents as compared to aprotic solvents has been explored to prevent the formation of mixtures of products in this reaction. Hydrated DBU is found to be superior to dry DBU, both for the selective synthesis and ease of isolation. In addition, use of DBU⋅H2O as a non‐nucleophilic mild base allowed us to synthesise 1‐(bromoethynyl)arenes via a reaction under solvent‐free conditions. Utilization of DBU⋅H2O as sole reagent also allowed us to isolate the products without column chromatographic purifications.magnified image

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