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==Heterocyclic spiro compounds==
[[Image:Spironolactone.svg | thumb | right | 200 px | A further example of a hetero-cyclic spiro compound, the on-market drug [[spironolactone]], where the two 5-membered rings ring in the upper right hand corner compose a spiro ring system. One of the rings is a [[lactone]] (i.e., contains as [[ester]] functional group).]]
Spiro compounds are considered heterocyclic if the spiro atom or any atom in either ring are not carbon atoms. Cases include the presence ofwith a spiro heteroatom such as boron, silicon, and nitrogen (but also other Group IVA [14] and other atom types) connecting the rings that have been observed or are underoften theoreticaltrivial study;<refto name=ElielEtal94/>{{rp|1139f}}{{citationprepare needed|date=February 2016}} moreover, thereas are also many cases where one or more heteroatoms appear in one or more of the rings that are joined at a carbon spiro atom (e.g., where 1Many oxygenborate spironolactonesesters andderived 2from oxygen/2glycols sulfurillustrate ketals/thioketalsthis are very common)case.{{citation needed|date=February 2016}}<ref name = RiosCSR12>{{cite journal | author doi= Rios, Ramon10.15227/orgsyn.087.0026 |title=Synthesis yearof =Spiroborate 2012Esters |from title1,2-Aminoalcohols, =Ethylene EnantioselectiveGlycol Methodologiesand forTriisopropyl theBorate: SynthesisPreparation of Spiro Compounds(S)-1-(1,3,2-Dioxaborolan-2-Yloxy)-3-Methyl-1,1-Diphenylbutan-2-Amine | journal =Organic [[Chemical Society Reviews]]Syntheses | volume date= 412010 | issue volume= 387 | pages page= 1060–1074 26| doi author=Viatcheslav 10.1039/C1CS15156HStepanenko, |Kun pmidHuang, =Margarita 21975423Ortiz-Marciales }}</ref>{{verification needed|date=February 2016}}
A common case is the presence of two atoms that are not carbon in one of the rings, with those two rings both attached to the spiro atom; indeed, often the earliest exposure of a chemist in training to a spiro compound is to a heterocyclic form, the [[acetal|ketal (acetal)]] formed in the [[protecting group|protection]] of ketones by [[diol]]s and [[dithiol]]s. An example of this is shown above, in the synthesis of the acetal 1,4-dioxaspiro[4.5]decane from [[cyclohexanone]] and [[ethanediol]]. In this case, because the four atoms attached to the spiro atom are not all carbons, the spiro atom is not a quaternary carbon. A further example of an [[acetal]] formed from a cyclic [[ketone]], except with a [[dithiol]], is the spiro compound [[spirapril]], which has a five-membered ring formed from [[1,2-ethanedithiol]]. Again, while the rings could be identical, in the heterocyclic case they are, again, almost always non-identical. Once again, the best-fit planes to each ring are generally non-coplanar to one another (i.e., the rings are not coplanar, despite appearing so in images). ▼<!--<ref name = RiosCSR12>{{cite journal | author = Rios, Ramon | year = 2012 | title = Enantioselective Methodologies for the Synthesis of Spiro Compounds | journal = [[Chemical Society Reviews]] | volume = 41 | issue = 3 | pages = 1060–1074 | doi = 10.1039/C1CS15156H | pmid = 21975423 }}</ref>{{verification needed|date=February 2016}}->
▲A particularly common casecases isare [[acetal|ketal (acetal)]] formed in the presence[[protecting group|protection]] of twoketones atomsby that[[diol]]s areand not[[dithiol]]s.<ref>{{cite carbonjournal in|doi=10.15227/orgsyn.093.0210 one|title=Preparation of the rings1, 5-Dioxaspiro[5.5]undecan-3-one with|date=2016 those|last1=Craig two|first1=Robert rings|last2=Smith both|first2=R. attachedC. to|last3=Pritchett the|first3=B. spiroP. atom;|last4=Estipona indeed,|first4=B. oftenI. the|last5=Stoltz earliest|first5=B. exposureM. of|journal=Organic aSyntheses chemist|volume=93 in|pages=210–227 training|pmid=28729749 to|pmc=5514842 a}}</ref><ref>{{cite spirojournal compound|doi=10.15227/orgsyn.071.0063 is|title=Stereocontrolled toPreparation aof heterocyclic3-Acyltetrahydrofurans form,from theAcid-Promoted [[acetal|ketalRearrangements of Allylic Ketals: ( acetal2S,3S) ]]-3-Acetyl-8-Carboethoxy-2,3-Dimethyl-1-Oxa-8-Azaspiro[4.5]Decane formed|journal=Organic inSyntheses the|date=1993 [[protecting|volume=71 group| protection]]page=63 of}}</ref><ref>{{cite ketonesjournal by|doi=10.15227/orgsyn.064.0073 [[diol]]s|title=Dichlorovinylation andof [[dithiol]]san Enolate: 8-Ethynyl-8-Methyl-1,4-Dioxaspiro[4. 5]Dec-6-Ene |journal=Organic Syntheses |date=1986 |volume=64 |page=73 }}</ref> An example of this is shown above, in the synthesis of the acetal 1,4-dioxaspiro[4.5]decane from [[cyclohexanone]] and [[ ethanediolglycol]]. In this case, because the four atoms attached to the spiro atom are not all carbons, the spiro atom is not a quaternary carbon. A further example of an [[acetal]] formed from a cyclic [[ketone]], except with a [[dithiol]], is the spiro compound [[spirapril]], which has a five-membered ring formed from [[1,2-ethanedithiol]]. Again, while the rings could be identical, in the heterocyclic case they are, again, almost always non-identical. Once again, the best-fit planes to each ring are generally non-coplanar to one another (i.e., the rings are not coplanar, despite appearing so in images).
==Nomenclature==
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