A common approach to pyrrolizidine and indolizidine alkaloids; formal synthesis of (−)-isoretronecanol, (−)-trachelanthamidine and an approach to the synthesis of (−)-5-epitashiromine and (−)-tashiromine

A common and short stereoselective route is described for the formal synthesis of pyrrolizidine alkaloids, (−)-isoretronecanol and (−)-trachelanthamidine. An approach to the synthesis of indolizidine alkaloids (−)-5-epitashiromine and (−)-tashiromine utilizing ring closing metathesis is also described starting from commercially available and inexpensive l-proline.     

First total synthesis of a new pyrrolizidine alkaloid, amphorogynine A

An efficient and stereodefined strategy is described for the first asymmetric synthesis of a new type of pyrrolizidine alkaloids, amphorogynine A and its 1-epi-isomer. The key 2,4-disubstituted pyrrolidine ring was constructed by elaboration of the chiral lactam derivative incorporating the d-malic acid-derived skeleton through asymmetric cis-allylation of the functionalized allysilane.     

Asymmetric synthesis of (+)-1-epiaustraline and attempted synthesis of australine

A diastereoselective synthesis of the pyrrolizidine alkaloid, (+)-1-epiaustraline has been achieved via a diastereoselective syn-dihydroxylation of a pyrrolo[1,2-c]oxazol-3-one precursor that was readily prepared by a RCM reaction. Attempts to extend this methodology to the synthesis of australine were not successful since the final pyrrolidine ring closure to produce the desired pyrrolizidine of the target molecule was not productive.     

A flexible stereospecific synthesis of polyhydroxylated pyrrolizidines from commercially available pyranosides

Nitrogen-containing sugar analogues, known as azasugars or iminosugars, such as polyhydroxylated piperdines, pyrrolidines, pyrrolizidines, and indolizidines, have the potential to become important therapeutic agents due to their ability to inhibit glycosidases. Synthetic pathways that are able to systematically produce a variety of these azasugars are eagerly sought after, since even minute structural or stereochemical changes often significantly alter the degree of inhibition. The synthesis of tetrahydroxylated pyrrolizidines 40 and 41 starting from methyl alpha-d-glucopyranoside is described and will be used as a template to develop syntheses of all the stereoisomers of polyhydroxylated pyrrolizidine 9 as well as other analogous bicyclic polyhydroxylated iminosugars. The key steps in this synthesis involve a one-pot conversion of a halopyranoside to a divinylamine by employing a simultaneous Zn reduction and reductive amination of the resulting aldehyde. After protection of the amine, a ring-closing metathesis results in a multifunctional eight-membered ring that then undergoes an internal S(N)2 cyclization to form an alkene-containing pyrrolizidine 33. Dihydroxylation of the alkene followed by hydrogenolysis of the benzyl protecting groups results in tetrahydroxylated pyrrolizidines 40 and 41.     

Titanium-Mediated Cyclization of omega-Vinyl Imides in Alkaloid Synthesis: Isoretronecanol, Trachelanthamidine, 5-Epitashiromine, and Tashiromine

A new method for the stereocontrolled synthesis of pyrrolizidine and indolizidine alkaloids by means of titanium-mediated cyclization of omega-vinyl imides is described. The general procedure involves treatment of readily available omega-vinyl imides 9 and 10 with 2.5 equiv of cyclopentylmagnesium chloride in the presence of ClTi(O-i-Pr)(3) (1.1 equiv) and subsequent stereoselective reduction of the N-acylaminal group. The cis and trans ring junction stereoisomers can be stereoselectively prepared by catalytic hydrogenation (H(2), PtO(2), EtOAc) and NaCNBH(3) reduction (TFA, MeOH), respectively. Finally, treatment of the resulting lactams with LAH or diborane afforded the target alkaloids 1-8 in good yields.     

Biomimetic Total Synthesis of (−)‐Penibruguieramine A Using Memory of Chirality and Dynamic Kinetic Resolution

The fully stereocontrolled total synthesis of (?)‐penibruguieramine A, a naturally occurring marine pyrrolizidine alkaloid, is described in this study for the first time. The key synthetic sequence is the biomimetic aldol reaction of the proline pentaketide amide. The principles of “memory of chirality” (MOC) and “dynamic kinetic resolution” (DKR) are applied to this reaction for the asymmetric synthesis using proline as the only chiral source. A mechanistic rationale is discussed for the excellent stereochemical outcome in a protic solvent environment.     

Partial reduction of pyrroles: application to natural product synthesis

The partial reduction of N-Boc pyrroles has been explored giving stereoselective routes to disubstituted pyrrolines in good yields and with excellent diastereoselectivities. A novel methodology has been developed to carry out reductive aldol reactions on 2-substituted N-Boc pyrroles; use of aldehydes under reductive aldol conditions gave the anti aldol product in good selectivity. This chemistry was used as the key transformation in a synthesis of omuralide, which was achieved in 13 steps and 14% overall yield. We also report a methodology for selectively forming either cis or trans 2,5-disubstituted pyrrolines via a partial reduction of an electron-deficient N-Boc pyrrole. The trans pyrroline formed using this route was utilized in the syntheses of the polyhydroxylated pyrrolizidine natural products hyacinthacine A1 and 1-epiaustraline. Further investigation has led to the development of routes to enantiopure substituted pyrroline compounds. This has been achieved via a chiral protonation approach using easily accessible chiral acids, such as ephedrine and oxazolidinones, to quench enolates formed during the partial reduction process. Alternatively, enzymatic desymmetrization of symmetrical diol compounds formed from the partial reduction products of substituted pyrroles is also reported. This leads to formation of both enantiomers of 2,2- and 2,5-disubstituted N-Boc pyrrolines in excellent ee and yields.     

Stereocomplementary Routes to Hydroxylated Nitrogen Heterocycles: Total Syntheses of Casuarine,Australine, and 7‐epi‐Australine

Addition of lithiated 1‐benzyloxyallene to a D ‐arabinose‐derived cyclic nitrone occurred with perfect diastereoselectivity furnishing a bicyclic 1,2‐oxazine derivative, which is an excellent precursor for pyrrolizidine alkaloids hydroxylated at C‐7 with optional configuration at this stereogenic center. Depending on the stage of the N? O bond cleavage and ring re‐closure, 7‐hydroxypyrrolizidines with 7R or 7S configuration were obtained, as a result of completely selective addition reactions occurring complementarily at the bottom or top face of the endocyclic C? C double bond in six‐ and five‐membered B rings, respectively. Applicability of these stereodivergent routes to obtain polyhydroxy pyrrolizidine alkaloids is demonstrated by the efficient syntheses of casuarine and australine as examples of the two classes of diversely configured 7‐hydroxypyrrolizidine alkaloids. An alternative synthesis of australine and two strategies for the preparation of 7‐epi‐australine are also reported, which demonstrate that the stereoselectivity of hydride reduction of an exocyclic C? O double bond is independent of the ring size, occurring preferentially from the top face either in a six‐ or five‐membered ring.     

First total synthesis and absolute configuration of naturally occurring (−)-hyacinthacine A7 and its (−)-1-epi-isomer

A convergent synthesis of the naturally occurring alkaloid (−)-hyacinthacine A₇, a glycosidase inhibitor of the pyrrolizidine class, is described. The homochiral starting material was tri-orthogonally protected DMDP 10, derived from d-fructose. Key steps of the synthesis were the carbon-chain lengthening at C(5′) in 10 to the α,β-unsaturated pyrrolidine ketone 12 and the one-pot construction of the bicyclic pyrrolizidine system of 13 and 14. Another key step was the partial inversion of the configuration at C(1) in 13 which led, after total deprotection, not only to the naturally occurring target molecule 9 but also to its (−)-1-epi-isomer 19.     

Total synthesis of (+)-hyacinthacine A2 based on SmI2-induced nitrone umpolung

[reaction: see text] A concise total synthesis of (+)-hyacinthacine A(2), a polyhydroxylated pyrrolizidine alkaloid, is described using our recently discovered inversion of the C-N bond polarity in nitrones. In the key step, the diastereoselective reductive coupling of a L-xylose-derived cyclic nitrone with ethyl acrylate allowed the assembly of the bicyclic core of the target molecule, by way of a tandem formation of the C-C and C-N bonds. The method opens a novel, short, and general route for the synthesis of other pyrrolizidine alkaloids.     

A concise and straightforward total synthesis of (+/-)-salinosporamide A, based on a biosynthesis model

A 14-step total synthesis of (+/-)-salinosporamide A (1), a potent inhibitor of the 20S proteasome isolated from the marine bacterium Salinospora tropica, is described. The synthesis is based on a diastereoselective intramolecular aldolisation of a substituted beta-keto amide intermediate, i.e. 13, derived from a beta-keto acid, viz. 21, and an alpha-amino malonate, leading to the pyrrolidinone ring 24 in the natural product. This synthetic approach closely mimics the origin of the pyrrolidinone ring in salinosporamide A in vivo. Another key feature of the total synthesis is a regioselective reduction of the malonate derivative 31 to the key aldehyde intermediate 32, using Super-hydride.     

Flexible strategy for the synthesis of pyrrolizidine alkaloids

A general strategy for the production of pyrrolizidine alkaloids is described, starting from intermediate (+)-9. The key features are diastereoselective dihydroxylation, inversion at the ring junction by hydroboration of an enamine, and ring closure to form the bicyclo ring system. This route is attractive because of its brevity and versatility; four natural products were prepared with differing stereochemistry and substitution patterns. Finally, this work allowed assignment of the absolute stereochemistry of 2,3,7-triepiaustraline and hyacinthacine A 7.     

Total synthesis of hyacinthacine A1 and 3-epi-hyacinthacine A1

[reaction: see text] Total synthesis of hyacinthacine A(1) and its epimer at C3 is described. The synthesis includes a stereocontrolled carboazidation of a chiral allylsilane as a key step. C-Si bond oxidation and reduction of the azide, with ring-closure, complete the total synthesis, which establishes the absolute configuration of 3.     

Total Synthesis of (±)‐Aspidophylline A

A total synthesis of aspidophylline A, a pentacyclic akuammiline‐type monoterpene indole alkaloid, is described. The synthesis features: 1) rapid access to a fully functionalized dihydrocarbazole through the desymmetrization of readily available 2‐allyl‐2‐(o‐nitrophenyl)cyclohexane‐1,3‐dione; 2) an intramolecular azidoalkoxylation of an enecarbamate to install both the furoindoline ring and the azido functionality; and 3) an intramolecular Michael addition for the construction of the 2‐azabicyclo[3.3.1]nonane ring system.     

The ketene thioacetal group as a cationic cyclization terminator. : A synthesis of the pyrrolizidine ring system.

A cationic cyclization to the pyrrolizidine ring system is reported in which a ketene thioacetal group serves as an efficient terminator for 5-membered ring formation. (±)-Supinidine is prepared using this methodology.     

Pyrrolizidine alkaloids

This review covers a range of pyrrolizidine alkaloids which have been isolated from various natural sources. The synthesis of necines and necic acids is described along with pharmacological and biological studies of pyrrolizidine alkaloids. The literature from July 2000 to June 2001 is reviewed, and 59 references are cited.     

A stereospecific synthesis of (±)-ferruginol

A stereospecific synthesis of (±)-ferruginol following the Robinson “ring extension” method has been described. The infra-red spectra of the benzoates of the synthetic and natural ferruginol are found to be identical.     

Stereoselective synthesis of the glycosidase inhibitor australine through a one-pot, double-cyclization strategy

[reaction: see text] A stereocontrolled, convergent synthesis of the alkaloid australine, a glycosidase inhibitor of the pyrrolizidine class, is described. The chiral starting materials were ketone 3, derived from L-erythrulose, and alpha-alkoxy aldehyde 4, prepared from L-malic acid. A key step of the synthesis was the highly stereoselective aldol reaction between 4 and a Z boron enolate derived from 3. Another key step was the one-pot construction of the bicyclic pyrrolizidine system by means of a three-step sequence of SN2 displacements induced by benzylamine on a trimesylate precursor.     

Enantioselective synthesis of saframycin A and evaluation of antitumor activity relative to ecteinascidin/saframycin hybrids

[formula: see text] A short synthesis of saframycin A is described which begins with a readily available intermediate previously utilized for the total synthesis of ecteinascidin 743. A key step in this synthesis is the use of 1-fluoro-3,5-dichloropyridinium triflate to oxidize a phenolic ring to a 1,4-benzoquinone unit while simultaneously cleaving a methoxymethyl ether of a different phenolic ring to the corresponding phenol (4-->5). The common intermediate (2) for the synthesis of saframycin A (1) and ecteinascidin 743 also allowed the synthesis of two hybrids of these structures (6 and 7). Whole cell bioassays for antitumor activity using lung, colon, melanoma, and prostate-derived tumor cell lines allowed a clear correlation of structure with biological activity in this series.     

15N nuclear magnetic resonance studies of [1-15N]nicotinamide adenine dinucleotides

The synthesis of [1-¹⁵N]nicotinamide adenine dinucleotide is described. Chemical shift data from ¹⁵N NMR studies are presented for the pyridine ring nitrogen of labeled NAD and related compounds. The results indicate a ¹⁵N label in the N-1 position to be highly sensitive to the redox-state of the pyridine moiety, with an upfield shift of over 100 ppm observed upon reduction of NAD⁺ to NADH. The feasibility of conducting ¹⁵N NMR studies of pyridine nucleotide binding to dehydrogenases is discussed.