Synthetic Study of (−)‐Norzoanthamine: Construction of the ABC Ring Moiety

Throw your hat in the ring : A highly diastereoselective synthesis of the ABC rings of (?)‐norzoanthamine has been achieved starting from the (?)‐Hajos–Parrish ketone (see scheme). Three asymmetric quaternary carbon centers on the C ring were constructed by a 1,4‐addition, and an intramolecular Diels–Alder reaction provided a trans‐decalin scaffold on the AB rings.

     

Enantioselective Nitroso‐Diels–Alder Reaction and Its Application for the Synthesis of (−)‐Peracetylated Conduramine A‐1

Cu^I‐catalyzed enantioselective nitroso‐Diels–Alder reactions (NDA reactions) of 2‐nitrosopyridine with various dienes are presented. The [CuPF₆(MeCN)₄]/Walphos‐CF₃ catalyst system is best suited to catalyze the NDA reaction of various dienes by using 2‐nitrosopyridine as a dienophile. In most of the cases studied, cycloadducts are obtained in quantitative yield with very good to excellent enantioselectivities. Based on DFT calculations, a model to explain the stereochemical outcome of the NDA reaction is presented. Finally, an efficient short synthesis of (?)‐peracetylated conduramine A‐1 by applying the enantioselective NDA reaction as a key step is described.     

Total Synthesis of (−)‐Norzoanthamine

No bones about it : (?)‐Norzoanthamine, a promising candidate for an anti‐osteoporotic drug, was the target of a total synthesis (see scheme). The final bisaminal formation with AcOH/H₂O gave the DEFG ring, while the cyclization precursor was prepared by installing the remaining bisaminal unit after oxidative cleavage of the cyclopentanol moiety.

     

Total Synthesis of (−)‐Caprazamycin A

Caprazamycin A has significant antibacterial activity against Mycobacterium tuberculosis (TB). The first total synthesis is herein reported and features a) the scalable preparation of the syn‐β‐hydroxy amino acid with a thiourea‐catalyzed diastereoselective aldol reaction, b) construction of a diazepanone with an unstable fatty‐acid side chain, and c) global deprotection with hydrogenation. This report provides a route for the synthesis of related liponucleoside antibiotics with fatty‐acid side chains.     

Enantiocontrolled Total Synthesis of (−)‐Mersicarpine

A racemic synthesis of mersicarpine ( 1 ) was achieved by the Mizoroki–Heck reaction and a DIBALH‐mediated reductive ring‐expansion reaction. Based on a first‐generation synthesis, a second‐generation enantiocontrolled total synthesis of (?)‐mersicarpine ( 1 ) was achieved by an 8‐pot/11‐step sequence in 21 % overall yield from commercially available 2‐ethylcyclohexanone. Subjection of a ketoester, which was prepared by an asymmetric Michael addition (according to the protocol by d’Angelo and Desmaële), and phenylhydrazine to modified Fischer indole conditions provided a six‐membered tricyclic indole. Benzylic oxidation and subsequent oxime formation provided a ketoxime, which was treated with diisobutylaluminum hydride (DIBALH) to construct the characteristic azepinoindole skeleton in good yield. In the DIBALH‐mediated reductive ring‐expansion reaction, gradually increasing the reaction temperature and in situ‐protection of the nitrogen in an oxygen‐sensitive azepinoindole with a benzyloxycarbonyl (Cbz) group were crucial for the high‐yielding process. With these methodologies, the short‐step and efficient synthesis of (?)‐mersicarpine was accomplished. Several synthetic efforts are also described.     

Total Synthesis of (+)‐Neomarinone

The first total synthesis of (+)‐neomarinone has been achieved by following a concise and convergent route using methyl (R)‐lactate and (R)‐3‐methylcyclohexanone as chiral building blocks. Key steps of the synthesis are the stereocontrolled formation of the two quaternary stereocenters by diastereoselective 1,4‐conjugate addition and enolate alkylation reactions, and the construction of the furanonaphthoquinone skeleton by regioselective Diels–Alder reaction between a 1,3‐bis(trimethylsilyloxy)‐1,3‐diene and a bromoquinone. The synthesis proves the relative and absolute stereochemistry of natural neomarinone.     

Bioinspired Synthesis of (−)‐PF‐1018

The combination of electrocyclizations and cycloadditions accounts for the formation of a range of fascinating natural products. Cascades consisting of 8π electrocyclizations followed by a 6π electrocyclization and a cycloaddition are relatively common. We now report the synthesis of the tetramic acid PF‐1018 through an 8π electrocyclization, the product of which is immediately intercepted by a Diels–Alder cycloaddition. The success of this pericyclic cascade was critically dependent on the substitution pattern of the starting polyene and could be rationalized through DFT calculations. The completion of the synthesis required the instalment of a trisubstituted double bond by radical deoxygenation. An unexpected side product formed through 4‐exo‐trig radical cyclization could be recycled through an unprecedented triflation/fragmentation.     

A Biomimetic Synthesis of (±)‐Basiliolide B

A highly diastereoselective and practical biomimetic total synthesis of (±)‐basiliolide B has been achieved through the study of the two proposed biosynthetic pathways (O‐methylation and O‐acylation) for the unprecedented 7‐methoxy‐4,5‐dihydro‐3H‐oxepin‐2‐one (C ring). The synthesis featured a cyclopropanation/ring opening strategy for establishing the stereogenic centers at C8 and C9, a biomimetic 2‐pyrone Diels–Alder cycloaddition for the synthesis of the ABD ring system, and finally a highly efficient biomimetic intramolecular O‐acylation for the C ring formation. This result provides an important perspective on the biosynthetic origin of the unprecedented 7‐membered acyl ketene acetal moiety of the C ring.     

Total Synthesis of (−)‐Enigmazole A

Total synthesis of (?)‐enigmazole A, a marine macrolide natural product with cytotoxic activity, has been accomplished. The tetrahydropyran moiety was constructed by means of a domino olefin cross‐metathesis/intramolecular oxa‐Michael addition of a δ‐hydroxy olefin. After coupling of advanced intermediates, the macrocycle was formed through gold‐catalyzed rearrangement of a propargylic benzoate, followed by ring‐closing metathesis of the resultant α,β‐unsaturated ketone.     

Total Synthesis of (−)‐Glaucocalyxin A

A practically useful method for the formation of the highly oxygenated bicyclo[3.2.1]octane ring system through Mn(OAc)₃‐mediated radical cyclization of alkynyl ketones was developed, which opens up a new avenue for the total synthesis of a number of highly oxidized diterpenoids. Application of this method enabled the first total synthesis of (?)‐glaucocalyxin A. Other salient features of the synthesis include a highly enantioselective conjugate addition/acylation cascade reaction, a Yamamoto aldol reaction, and an intramolecular Diels–Alder reaction to assemble the A/B ring system.     

Total Synthesis of (−)‐Nakadomarin A

A highly efficient 12‐step synthesis of the marine alkaloid (?)‐nakadomarin A has been accomplished. The key advanced intermediate, a tetracyclic ketone derivative, was constructed in just seven steps using a sequence that includes an asymmetric Pauson–Khand reaction, an Overman rearrangement reaction, a ring‐closing metathesis reaction, and an amination reaction. Late introduction of the furan ring during the synthesis of (?)‐nakadomarin A means that the key tetracyclic ketone derivative has the potential to serve as an advanced intermediate for the synthesis of related marine alkaloids.     

A General Entry to Antifeedant Sesterterpenoids: Total Synthesis of (+)‐Norleucosceptroid A, (−)‐Norleucosceptroid B,and (−)‐Leucosceptroid K

The first asymmetric total synthesis of the antifeedant terpenoids (+)‐norleucosceptroid A, (?)‐norleucosceptroid B, and (?)‐leucosceptroid K has been accomplished. This highly concise synthetic route was guided by our efforts to develop a platform for the collective synthesis of a whole family of antifeedant natural products. The synthesis features a Hauser–Kraus‐type annulation followed by an unprecedented, highly efficient intramolecular dilactol aldol‐type condensation reaction to produce the 5,6,5 skeleton. The developed synthetic route proceeds for norleucosceptroid A and B in 16 steps (longest linear sequence) from known compounds.     

Total Synthesis of (+)‐Sarcophytin

A total synthesis of the cembranoid (+)‐sarcophytin is presented, featuring a Diels–Alder cycloaddition of an enone as the dienophile with an ester‐derived dienoate. The study highlights a peculiar geometric preference for the Z dienoate to furnish the cycloadduct. The endgame involves a reaction cascade, including lactone opening, alcohol oxidation, and ketone epimerization to complete an efficient synthesis. A salient feature of the synthesis is the resulting reassignment of the absolute configuration, which corrects the previously reported nominal structure.