Stereocontrolled Construction of 6 · 5 · 6 and 5 · 5 · 6 Ring Systems by Radical Tandem Cyclization

Experimental and Theoretical Study on the Regioselective Radical Cyclization Reactions of 1-(o-Alkenylaryl)-2-amido-1-ketones for the Construction of Indeno[2,1-d][1,3]oxazin-9-ones

A simple and efficient two-step method for the construction of novel 2,4,9a-trisubstituted-4a,9a-dihydroindeno[2,1-d][1,3]oxazin-9-ones has been developed. The NHC-catalyzed aza-benzoin reaction of o-alkenyl benzaldehydes with N-acylarylimines afforded 1-(o-alkenylaryl)-2-amido-2-aryl-1-ethanones, which underwent regioselective 5-exo-trig radical cyclization to furnish the three-ring-fused heterocyclic products, generally in good yields. The synthetic method displayed good tolerance toward the nature of substituents, substitution pattern, and steric hindrance of o-alkenyl benzaldehydes. Based on this method, the synthesis of unprecedented dihydrobenzo[6,7]indeno[2,1-d][1,3]oxazin-7-ones and dihydropyrido[2',3':3,4]cyclopenta[1,2-d][1,3]oxazin-9-ones has been achieved by employing o-alkenylnaphthaldehyde and o-alkenylnicotinaldehyde as substrates. The regioselectivity between 5-exo-trig and 6-endo-trig radical cyclization reactions of different 1-(o-alkenylaryl)-2-amido-2-aryl-1-ethanones were elucidated with DFT calculations.

Development of an Enantioselective Synthesis of (-)-Euonyminol

We detail the development of the first enantioselective synthetic route to euonyminol (1), the most heavily oxidized member of the dihydro-β-agarofuran sesquiterpenes and the nucleus of the macrocyclic alkaloids known as the cathedulins. Key steps in the synthetic sequence include a novel, formal oxyalkylation reaction of an allylic alcohol by [3 + 2] cycloaddition; a tandem lactonization-epoxide opening reaction to form the trans-C2-C3 vicinal diol residue; and a late-stage diastereoselective trimethylaluminum-mediated α-ketol rearrangement. We report an improved synthesis of the advanced unsaturated ketone intermediate 64 by means of a 6-endo-dig radical cyclization of the enyne 42. This strategy nearly doubled the yield through the intermediate steps in the synthesis and avoided a problematic inversion of stereochemistry required in the first-generation approach. Computational studies suggest that the mechanism of this transformation proceeds via a direct 6-endo-trig cyclization, although a competing 5-exo-trig cyclization, followed by a rearrangement, is also energetically viable. We also detail the challenges associated with manipulating the oxidation state of late-stage intermediates, which may inform efforts to access other derivatives such as 9-epi-euonyminol or 8-epi-euonyminol. Our successful synthetic strategy provides a foundation to synthesize the more complex cathedulins.

Stereo- and Regioselective Synthesis of Tricyclic Spirolactones by Diastereoisomeric Differentiation of a Collective Key Precursor

A general, parallel, and collective synthesis of 5/5/5- and 5/5/6-ring fusion topologies of tricyclic spiranoid lactones through the controlled cyclizations of easily accessible, common key precursors is described. The rapid composition of key cycloalkyl methylene precursors yielded an assembly of bicyclic diastereoisomeric iodolactones, which were individually converted into a wide range of tricyclic, angularly fused spiranoid lactones in a regioselective and stereodirected fashion through the diastereoisomeric differentiation of a collective key precursor. The critical stereochemical assignment of the bicyclic starting materials, as well as the tricyclic targets, was confirmed by X-ray crystal structure determination.

A neighboring group participation strategy: direct and highly diastereoselective synthesis of 2-substituted and 2,2-bisubstituted perhydrofuro[2,3-b]pyran derivatives

A highly diastereoselective synthesis method for 2-substituted and 2,2-bisubstituted perhydrofuro[2,3-b]pyran derivatives from 2-C-branched sugars via a neighboring group participation strategy was developed.