Recent Progress in the Synthesis of Silaspiranes

Silaspiranes bearing a spiro-silicon center are promising ring frameworks for the synthesis of novel spirocyclic molecules possessing unique properties. Development of efficient methods towards these ring structures has therefore attracted considerable attentions of synthetic chemists. This minireview highlights the representative advances in the field, and is categorized into four parts according to the ring formation strategies: cyclization, annulation, ring expansion and cycloaddition.

(1RS,2RS,6RS)-2-(6-Amino-9H-purin-9-yl)-8-azaspiro[5.6]dodec-10-en-1-ol Dihydrochloride

The title compound (1RS,2RS,6RS)-2-(6-Amino-9H-purin-9-yl)-8-azaspiro[5.6]dodec-10-en-1-ol dihydrochloride was synthesized for the first time in two steps, the first of which was a regioselectivity of epoxide ring-opening reaction under the action of adenine, yielding N-tert-butoxycarbonyl-((1RS,2RS,6RS)-2-(6-amino-9H-purin-9-yl)-8-azaspiro[5.6]dodec-10-en-1-ol. By treating the latter with a saturated methanolic solution of hydrogen chloride, it was possible to obtain ((1RS,2RS,6RS)-2-(6-Amino-9H-purin-9-yl)-8-azaspiro[5.6]dodec-10-en-1-ol dihydrochloride. The features of the molecular and crystal structure of ((1RS,2RS,6RS)-2-(6-Amino-9H-purin-9-yl)-8-azaspiro[5.6]dodec-10-en-1-ol dihydrochloride are discussed based on X-ray diffraction studies. The product overall yield was 40% out of two steps and after purification by column chromatography and recrystallization. The product was characterized by 1H-NMR, 13C-NMR, IR spectroscopy, HRMS and X-ray.

A Type of Structurally Adaptable Aromatic Spiroketal Based Chiral Diphosphine Ligands in Asymmetric Catalysis

ConspectusWhile spectacular successes have been achieved in homogeneous catalysis with the use of achiral diphosphine ligands featuring a wide natural bite angle, such as XantPhos, chiral diphosphines that can induce a large P-M-P bite angle in their transition metal complexes are conspicuously less explored in asymmetric catalysis, probably due to the challenges in the identification and efficient construction of a suitable chiral backbone. In the past decade, a highly efficient synthesis of chiral aromatic spiroketals and the corresponding diphosphine ligands (SKPs) has been developed in this group.Based on a one-pot catalytic tandem double asymmetric hydrogenation-spiroketalization ring-closure reaction sequence, these SKP ligands featuring an extraordinarily long P···P distance and a flexible backbone have been readily prepared in large scale. Remarkably versatile coordination modes have been found in the complexes of SKP with some catalysis-relevant transition metals, for example, Pd, Cu, Au, and Rh. Whereas SKP enforces an unusually large bite angle in [Pd(SKP)Cl₂] and [Cu(SKP)Cl] complexes (160.1° and 132.8°, respectively), it also allows for a bimetallic Au-Au interaction (3.254 Å) in the complex of [Au₂(SKP)Cl₂] or a square-planar coordination geometry for the [Rh(SKP)(cod)]SbF₆ complex. Such an adaptable nature of SKP ligands for transition metal coordination has profound consequences in homogeneous asymmetric catalysis, as demonstrated by their unique performance in several types of catalytic asymmetric reactions. One of the most exciting examples is SKP/Pd-catalyzed asymmetric allylic amination of Morita-Baylis-Hillman (MBH) adducts, in which SKP/Pd complexes demonstrated excellent control of regio- and enantioselectivities and exhibited exceptionally high efficiency (with a TON up to 4750) in the catalysis. SKP ligands have also found a diversity of successful applications in Cu-, Au-, or Rh-catalyzed asymmetric reactions, further attesting their wide utilities in asymmetric catalysis. Overall, this class of readily accessible SKP ligands featuring a chiral aromatic spiroketal skeleton have demonstrated unique adaptable structures in a variety of transition metal complexes and provided outstanding performance in some difficult asymmetric transformations. The works delineated herein would be expected to stimulate further research efforts on the application of this type of chiral ligand and to provide useful clues in the design of new chiral diphosphine ligands with adaptable bite angles for transition metal catalyzed asymmetric reactions.

Development of a C2-Symmetric Chiral aza Spirocyclic Diol

A C₂-symmetric chiral spirocyclic diol aza-SPINOL containing a spirooxindole scaffold has been designed, synthesized, and optically resolved. The product could be synthesized on gram scale in an overall yield of 22%. Moreover, elaborations of aza-SPINOL to other chiral ligands as well as the preliminary investigation of the related bisphosphine ligand in the desymmetrization of bisallylic amide were reported.

Synthesis and application of a new chiral monodentate spiro phosphoramidite ligand based on hexamethyl-1,1'-spirobiindane backbone in asymmetric hydroamination/arylation of alkenes

The design and synthesis of a new chiral monodentate spiro phosphoramidite ligand based on a hexamethyl-1,1'-spirobiindane scaffold has been accomplished. The ligand could serve as an elegant chiral monodentate ligand in the Pd-catalyzed asymmetric hydroamination/arylation of alkenes leading to chiral imidazolidin-2-ones with good enantioselectivities.

Chiral Cyclohexyl-Fused Spirobiindanes: Practical Synthesis, Ligand Development, and Asymmetric Catalysis

1,1'-Spirobiindane has been one type of privileged skeleton for chiral ligand design, and 1,1'-spirobiindane-based chiral ligands have demonstrated outstanding performance in various asymmetric catalysis. However, the access to enantiopure spirobiindane is quite tedious, which obstructs its practical application. In the present article, a facile enantioselective synthesis of cyclohexyl-fused chiral spirobiindanes has been accomplished, in high yields and excellent stereoselectivities (up to >99% ee), via a sequence of Ir-catalyzed asymmetric hydrogenation of α,α'-bis(arylidene)ketones and TiCl₄ promoted asymmetric spiroannulation of the hydrogenated chiral ketones. The protocol can be performed in one pot and is readily scalable, and has been utilized in a 25 g scale asymmetric synthesis of cyclohexyl-fused spirobiindanediol (1 S,2 S,2' S)-5, in >99% ee and 67% overall yield for four steps without chromatographic purification. Facile derivations of (1 S,2 S,2' S)-5 provided straightforward access to chiral monodentate phosphoramidites 6a-c and a tridentate phosphorus-amidopyridine 11, which were evaluated as chiral ligands in several benchmark enantioselective reactions (hydrogenation, hydroacylation, and [2 + 2] reaction) catalyzed by transition metal (Rh, Au, or Ir). Preliminary results from comparative studies showcased the excellent catalytic performances of these ligands, with a competency essentially equal to the corresponding well-established privileged ligands bearing a regular spirobiindane backbone. X-ray crystallography revealed a close resemblance between the structures of the precatalysts 20 and 21 and their analogues, which ultimately help to rationalize the almost identical stereochemical outcomes of reactions catalyzed by metal complexes of spirobiindane-derived ligands with or without a fused cyclohexyl ring on the backbone. This work is expected to stimulate further applications of this type of readily accessible skeletons in development of chiral ligands and functional molecules.

Synthesis and application of a new hexamethyl-1,1′-spirobiindane-based chiral bisphosphine (HMSI-PHOS) ligand in asymmetric allylic alkylation

A new class of hexamethyl-1,1′-spirobiindane-based chiral bisphosphine ligand was synthesized and used in Pd-catalyzed asymmetric allylic alkylation reactions.

Metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides

In this review recent advances in the metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides are highlighted, covering references from the period 2000 until August 2018.

Palladium-catalyzed asymmetric allylic amination: enantioselective synthesis of chiral α-methylene substituted β-aminophosphonates

Palladium catalyzed asymmetric allylic amination of 2-diethylphosphonate-substituted allylic acetates in the presence of SKP ligands afforded the corresponding chiral β-aminophosphonates in good yields, high regioselectivities, and excellent enantioselectivities.

Enantioselective synthesis of tetrahydrocyclopenta[b]indole bearing a chiral quaternary carbon center via Pd(ii)–SPRIX-catalyzed C–H activation

Pd(ii)–SPRIX promotes the first highly enantioselective cyclization of 3-alkenylindoles into the corresponding indoles bearing a chiral quaternary carbon center via C–H activation and an assisting effect by the allyl substituent.

Catalytic Enantioselective Synthesis of C1 - and C2 -Symmetric Spirobiindanones through Counterion-Directed Enolate C-Acylation

A catalytic enantioselective route to C1 - and C2 -symmetric 2,2'-spirobiindanones has been realized through an intramolecular enolate C-acylation. This reaction employs a chiral ammonium counterion to direct the acylation of an in situ generated ketone enolate with a pentafluorophenyl ester. This reaction constitutes the first example of a direct catalytic enantioselective C-acylation of a ketone and provides an efficient and highly enantioselective route to axially chiral spirobiindanediones. These products can be diastereoselectively derivatized, offering access to a range of functionalized spirocyclic architectures.

Structural Features and Asymmetric Environment of i-Pr-SPRIX Ligand

Novel chiral diisopropyl spiro bis(isoxazoline) ligands, anti-i-Pr-SPRIX and syn-i-Pr-SPRIX, were designed and synthesized. Their catalytic utility, X-ray crystallographic analyses, and complexation studies demonstrated the structural features of tetraisopropyl spiro bis(isoxazoline) ligand, i-Pr-SPRIX, which is a prominent ligand in various enantioselective Pd catalytic processes: All i-Pr groups work in collaboration to create an effective asymmetric environment.

Spiro-fused six-membered N-heterocyclic carbene: a new scaffold toward unique properties and activities

A six-membered N-heterocyclic carbene fused with a spiro-scaffold is designed. The new NHC shows stronger σ-donation ability than typical 5-membered NHCs. This property leads to interesting reactivities of this spiro-fused six-membered NHC. For example, the NHC-BF3 Lewis pair complex can be readily prepared by using LiBF4 as the BF3 source, or through a direct bond-reconstruction of the tetrafluoroborate salt NHC·HBF4.

Pd-catalyzed enantioselective intramolecular α-arylation of α-substituted cyclic ketones: facile synthesis of functionalized chiral spirobicycles

Synthesis of chiral spirocyclic ketones was accomplishedviathe Pd-catalyzed intramolecular α-arylation of α-substituted cyclic ketones. The obtained spirocyclic ketone could be converted into an acid–base organocatalyst.

Palladium(ii)-catalyzed intramolecular carboxypalladation–olefin insertion cascade: direct access to indeno[1,2-b]furan-2-ones

An efficient intramolecular carboxypalladation–olefin insertion cascade was developed for the synthesis of indeno[1,2-b]furan-2-ones.

Pd(II)-SDP-catalyzed enantioselective 5-exo-dig cyclization of γ-alkynoic acids: application to the synthesis of functionalized dihydofuran-2(3H)-ones containing a chiral quaternary carbon center

The Pd(II)-SDP-catalyzed first enantioselective intramolecular cyclization of α,α-disubstituted γ-alkynoic acids is described. This 5-exo-dig cyclization afforded dihydrofuran-2(3H)-ones bearing a chiral quaternary carbon center in excellent yields with enantioselectivities up to 71%. A mechanism involving palladium(II) species is proposed to rationalize the outcome of the reaction.