33) Spirocycles-Organic and Biomolecular Chemistry, 2003, 14 (10), 1363-1370..pdf (original) (raw)
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Organic & Biomolecular Chemistry, 2003
In the presence of allyl tri-n-butyltin-AIBN, cyclopropylmethyl bromides/xanthates undergo ring-opening reaction with concomitant formation of geminal diallyl derivatives in good yields. The ring closing metathesis reactions on geminal diallyl derivatives with Grubbs' catalyst provided spirocyclopentenyl products. Combination of these two methodologies has been applied to the synthesis of mono-, bis-cyclopentyl-carbohydrates as well as spirocyclopentylproline derivatives. Scheme 1 Scheme 2 Reagents and conditions: (a) allyltri-n-butyltin, C 6 H 6 , AIBN, 80 ЊC, 76%. O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 3 6 6 -1 3 7 3 T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3 Scheme 4 Reagents and conditions; (a) methallyltributyltin, C 6 H 6 , AIBN, 80 ЊC, 12 h, 52%. O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 3 6 6 -1 3 7 3
ChemInform Abstract: Glycosylidene Carbenes. Part 3. Synthesis of Spirocyclopropanes
ChemInform, 2010
ChemInform Abstract The phenols (II) are treated with compounds such as (I) and (V) in order to elucidate the influence of the acidity of phenols on the yields, diastereoselectivity and regioselectivity of their glycosidation. It is shown, that differences in the kinetic acidities of glycosyl acceptors possessing more than one OH-group serve as the basis for a regioselective reaction which does not require protective groups (cf. (I) → (-)-(VIII) + (+)-(IX)) and that steric hindrance is relative unimportant for the synthesis of glucosides (cf. reaction of I) with (X)). O-Aryl glycosides possess a variety of biological activities.
Synthesis of spiro-cyclics via ring-closing metathesis
Arkivoc, 2003
Various spiro-cyclic systems were synthesized by utilizing ring-closing metathesis as a key step. The required dialkylated starting materials were prepared from various 1,3-diketones or substrates containing an active methylene group.
Synthesis of unusually strained spiroheterocyclic ring systems and their exploits in synthesis
Tetrahedron, 2009
phosphonium ylides. However, an appropriate directing group (i.e. ether) was required because of the modest diastereoselectivity observed. After this initial report, several other methods were devised for the synthesis of oxaspiro[2.2]pentanes, which include addition of lithiated bromocyclopropanes 5 or diazocompounds to ketones,6 reaction of singlet oxygen with bicyclopropylidene,7 , 8 selenone additions to ketones, 9 and the addition of cyclopropyl sulfur ylides to carbonyl compounds. By far the most widely used and arguably the most diastereoselective synthesis of oxaspiro[2.2]pentanes remains the addition of cyclopropyl sulfoxonium ylides to carbonyl compounds. 12, 13 A series of elegant studies revealed that ylide addition proceeds via equatorial attack at the carbonyl carbon in cyclic compounds. While this process is highly diastereoselective, an enantioselective process has not been developed. Although Johnson reported chiral sulfoximine ylide additions to α,β-unsaturated carbonyl compounds, the racemic counterpart performed poorly with simple carbonyl compounds as in the reaction of cyclohexanone 3 with sulfoximine ylide 4, thus the chiral series was not attempted (Scheme 2). 14 Oxaspiro[2.2]pentanes display two primary modes of reactivity. One mode, leading to a cyclobutanone, appears to be driven by the release of the strain energy of the ring system (Scheme 3).12 However, as cyclobutanes possesses essentially the same amount of strain energy as cyclopropanes (26 kcal/mol and 27 kcal/mol, respectively) due to the eclipsing interactions of the substituents on a cyclobutane, the rearrangement is driven not only by the release of ring strain of both the starting epoxide and the cyclopropane but also by the formation of a C=O bond. The ring expansion of a cyclopropane to a cyclobutane proceeds with oxaspiro[2.2]pentane 6 when appropriate stabilizing groups are present. Ionization of the epoxide, either by addition of a Lewis acid or thermal induction, yields a highly stabilized intermediate cyclopropyl carbinyl cation 7. This type of cation is uniquely stabilized due to the enhanced π-character of the σ-bonds in cyclopropanes, which consequently permits a pinacol like rearrangement to cyclobutanone 8. This process is favorable due to the release of ring strain of the starting epoxide and cyclopropane in addition to the formation of a C=O bond.
Synthesis, structure and enzymatic evaluation of new spiro oxathiazole sugar derivatives
Tetrahedron, 1994
On treatment with N-bromosuccinimide under irradiation in refluxing carbon tetrachlotide, 2,3,4.6-tetra-O-acetyl-l-S-(Z)-benzhydroximoyl-l-thio-D-glucopyranose 1 and various analogs yielded new Spiro anomeric oxathiazole derivatives in-60% total yield. After deacetylation, the tested major l(S) epimers were found good competitive inhibitors of emulsin g-D-glucosidase whereas a l(R) counterpart had no effect on the enzyme. In connection with the outstanding development of radical-mediated methodologies over the last decade, a number of new synthetic modifications involving the anomeric centre of oligosaccharides has been proposed. A large array of methods is now available for the generation of carbon-centered anomeric radicals1 by either inter-or intramolecular processes. In this latter case, especially designed glycosides must be used to promote, under appropriate conditions, the formation of activated species or free-radical transients able to cleave homolytically anomeric carbon-hydrogen bonds. Pioneering studies have shown that such homolyses occur via six-membered cyclic transition states involving the activated species and the hydrogen atom to be abstracted, in either the intramolecular cyclization of hypohalidesz or the Barton reaction 3. Hence, only glycosides in which suitable functional groups are located at the y carbon atom of the aglycon are good candidates for efficient generation of anomeric radicals via lJ-shifts. Afterwards, a cascade of steps ensues which ends up in ring-closure by either CC or CO bond formation as illustrated by the high-yielding syntheses of new Spiro derivatives from either 3'-oxoalkyl-g-D-glucopyranosides4 or their 2'-hydroxyalkyl counterparts 5.6. The recently described reductive cyclization of iodoepoxy-g-D-glucopyranosides7 in the presence of tributyltin hydride used the same concept and showed again the high a-stereoselectivity of the ring-closures in sugar derivatives displaying the 4Cl-D chair conformation.
Two directional approach to spirocyclic ethers via Grignard reaction and ring-closing metathesis
2016
Diverse and intricate bis-spirocycles have been assembled via a Grignard reaction and ring-closing metathesis (RCM) as key steps. Readily available starting materials have been used and mild reaction conditions employed to generate a variety of intricate spirocyclic ethers containing pyran ring system. The scope of this methodology has then been expanded by synthesizing a spiro-dihydrofuran derivative.