Akshat H. Rathi

Recent developments in methodology for the direct oxyamination of olefins.

1,2-Amino alcohols are high-value, versatile functional groups that are found in scores of biologically active molecules and other interesting synthetic targets such as ligands and auxiliaries. Given their prominent position within organic compounds of import, it is no surprise to note that many routes have been developed to access this motif and there are many different starting points from which a synthetic chemist might embark on a synthesis. However, one particular approach stands out from the others, and this is the direct conversion of an alkene to a vicinal amino alcohol derivative (oxyamination). Research in this field has been particularly active in recent years and many interesting new methodologies have been reported. The purpose of this review is to give the reader a tour of the methods that have emerged in the last few years so one can appreciate the myriad of different metals and reagents that can accomplish the oxyamination of alkenes. There are still many challenges to be overcome and, herein, we also outline the areas that are ripe for further development and which bode well for the future.

Heteroaromatic Synthesis via Olefin Cross-Metathesis: Entry to Polysubstituted Pyridines

The olefin cross-metathesis reaction provides a rapid and efficient method for the synthesis of α,β-unsaturated 1,5-dicarbonyl derivatives which then serve as effective precursors to mono-tetrasubstituted pyridines. Manipulation of the key 1,5-dicarbonyl intermediate allows access to pyridines with a wide range of substitution patterns. An extension of this methodology facilitates the preparation of pyridines embedded within macrocycles, as exemplified by an efficient synthesis of (R)-(+)-muscopyridine. High levels of regiocontrol, short reaction sequences, and facile substituent variation are all notable aspects of this methodology.

Total synthesis of the antitumor antibiotic (±)-streptonigrin: first- and second-generation routes for de novo pyridine formation using ring-closing metathesis.

The total synthesis of (±)-streptonigrin, a potent tetracyclic aminoquinoline-5,8-dione antitumor antibiotic that reached phase II clinical trials in the 1970s, is described. Two routes to construct a key pentasubstituted pyridine fragment are depicted, both relying on ring-closing metathesis but differing in the substitution and complexity of the precursor to cyclization. Both routes are short and high yielding, with the second-generation approach ultimately furnishing (±)-streptonigrin in 14 linear steps and 11% overall yield from inexpensive ethyl glyoxalate. This synthesis will allow for the design and creation of druglike late-stage natural product analogues to address pharmacological limitations. Furthermore, assessment of a number of chiral ligands in a challenging asymmetric Suzuki–Miyaura cross-coupling reaction has enabled enantioenriched (up to 42% ee) synthetic streptonigrin intermediates to be prepared for the first time.

Synthesis and Phytotoxic Activity of New Pyridones Derived from 4-Hydroxy-6-Methylpyridin-2(1H)-one

Commercial dehydroacetic acid was converted into 4-hydroxy-6-methylpyridin-2(1H)-one (3), which was then condensed with several aliphatic aldehydes to produce seven new title compounds in variable yields (35–92%). Reaction of 3 with α,β-unsaturated aldehydes resulted in the formation of condensed pyran derivatives 4g’ and 4h’. A mechanism is proposed to explain the formation of such compounds. The effects of all methylpyridin-2(1H)-one derivatives on the development of the dicotyledonous species Ipomoea grandifolia and Cucumis sativus and the monocotyledonous species Sorghum bicolor were evaluated. At the dose of 6.7 × 10-8 mol a.i./g substrate the compounds showed some phytotoxic selectivity, being more active against the dicotyledonous species. These compounds can be used as lead structures for the development of more active phytotoxic products.

Intramolecular hydride addition to pyridinium salts: new routes to enantiopure dihydropyridones.

The transformation of a simple disubstituted pyridine into a pyridinium ion bearing an exocyclic hydroxyl group, protected as a silane, enabled an intramolecular hydride transfer reaction to take place when fluoride was used as a nucleophile. The addition was both regio- and stereoselective and enabled the formation of enantiopure dihydropyridones when enantiopure pyridine derivatives were used in this sequence. The heterocyclic products contain ample functionality for further elaboration reactions and subsequent derivatization.

Tethered Aminohydroxylation: Synthesis of the β-Amino Acid of Microsclerodermins A and B

The utility of the tethered aminohydroxylation (TA) has been demonstrated by synthesis of the complex β-amino acid residue of microsclerodermins A and B. The TA provided a regio- and stereoselective functionalization of a complex homoallylic alcohol. The route includes late-stage introduction of the aliphatic side chain via a cuprate addition and cross metathesis, a tactic designed to render the synthesis applicable to other microsclerodermins.