Expanding Radical SAM Chemistry by Using Radical Addition Reactions and SAM Analogues

Radical S‐adenosyl‐l ‐methionine (SAM) enzymes utilize a [4Fe‐4S] cluster to bind SAM and reductively cleave its carbon–sulfur bond to produce a highly reactive 5′‐deoxyadenosyl (dAdo) radical. In almost all cases, the dAdo radical abstracts a hydrogen atom from the substrates or from enzymes, thereby initiating a highly diverse array of reactions. Herein, we report a change of the dAdo radical‐based chemistry from hydrogen abstraction to radical addition in the reaction of the radical SAM enzyme NosL. This change was achieved by using a substrate analogue containing an olefin moiety. We also showed that two SAM analogues containing different nucleoside functionalities initiate the radical‐based reactions with high efficiencies. The radical adduct with the olefin produced in the reaction was found to undergo two divergent reactions, and the mechanistic insights into this process were investigated in detail. Our study demonstrates a promising strategy in expanding radical SAM chemistry, providing an effective way to access nucleoside‐containing compounds by using radical SAM‐dependent reactions.     

Oxidative Cleavage of C=S and P=S Bonds at an AlI Center: Preparation of Terminally Bound Aluminum Sulfides

The treatment of cyclic thioureas with the aluminum(I) compound NacNacAl ( 1 ; NacNac=[ArNC(Me)CHC(Me)NAr]^?, Ar=2,6‐Prⁱ₂C₆H₃) resulted in oxidative cleavage of the C=S bond and the formation of 3 and 5 , the first monomeric aluminum complexes with an Al=S double bond stabilized by N‐heterocyclic carbenes. Compound 1 also reacted with triphenylphosphine sulfide in a similar manner, which resulted in cleavage of the P=S bond and production of the adduct [NacNacAl=S(S=PPh₃)] ( 8 ). The Al=S double bond in 3 can react with phenyl isothiocyanate to furnish the cycloaddition product 9 and zwitterion 10 as a result of coupling between the liberated carbene and PhN=C=S. All novel complexes were characterized by multinuclear NMR spectroscopy, and the structures of 5 , 9 , and 10 were confirmed by X‐ray diffraction analysis. The nature of the Al=S bond in 5 was also probed by DFT calculations.     

KHCO3‐ and DBU‐Promoted Cascade Reaction to Synthesize 3‐Benzyl‐2‐phenylquinolin‐4(1 H)‐ones

A novel and convenient one‐pot route for the synthesis of 3‐benzyl‐2‐phenylquinolin‐4(1 H)‐ones has been developed under transition‐metal‐free conditions. This new strategy features high yield and good functional group tolerance. In addition, a proposed mechanism has been confirmed for this reaction.     

Neat total synthesis of six monoterpenic alkaloids of the actinidine series

A concise enantiopure synthesis of six monoterpenic alkaloids of the actinidine series possessing a cyclopenta[c]pyridine skeleton, (+)-deoxyrhexifoline (4), (+)-boschniakinic acid (5), (+)-boschniakine (6), (−)-plantagonine (7), (−)-indicaine (8) and (−)-tecostidine (9) is reported starting with the chiral precursor 3-bromo-5-((4R)-phenyloxazolin-2-yl)pyridine (10). It involves a C-4 regioselective connection of a butene appendice and an intramolecular 5-exo-trig Heck annulation sequence followed by hydrogenation of the exocyclic alkene. Mixture of (3R)- and (3S)-7-((4R)-phenyloxazolin-2-yl)cyclopenta[c]pyridines was separated by HPLC before being transformed into enantiopure natural products (4-9) by modification of the oxazoline group.     

Evaluation of phenylorganotellurium compounds as radical precursors in dialkylzinc-mediated radical addition to CN double bonds

Diethylzinc-mediated radical addition to CN bonds was investigated in the presence of phenylorganotellurium compounds as radical precursors. As group transfer agents, secondary alkyl phenyl tellurides were shown to be about twice as reactive as the corresponding alkyl iodides towards ethyl radical. Their use was proven to be advantageous regarding both chemoselectivity and yield. The replacement of diethylzinc by dimethylzinc, offers no advantage in these reactions.     

An enantioselective total synthesis of pinnaic acid

An enantioselective and convergent total synthesis of marine natural product pinnaic acid has been achieved. Our general synthetic approach is featured with an asymmetric hydrogenation of racemic γ-keto ester 3, a diastereoselective methylation on the α-methylene of the (1R,5R)-lactone 4, and a diastereoselective Michael addition of the tertiary nitro cyclopentane. The central azaspiro[4.5]decane was constructed utilizing reductive cyclization of the δ-nitroketone followed by highly stereoselective reduction of the cyclic imine with NaBH₄. Ultimately, successive use of triethyl-2-phosphonopropionate and Heathcock's phosphorane 18 to elaborate C5 and C13 side chains completed the total synthesis of pinnaic acid.