<MultiModal>![O=C1[C@](C)(CC(OCC)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O>CC#CO[Li]>[*]](/media/uploads/2a62e453-ef96-4843-ad19-79a3516a6a38.png)</MultiModal>

The product $\bf{A}$ generated by the above reaction contains three five-membered rings. The formation of this product sequentially goes through two key electroneutral intermediates $\bf{B}$ and $\bf{C}$.

It is known that $\rm{LiOEt}$ is produced simultaneously with the generation of $\bf{B}$.

Which of the following statements is correct:
<MultiModal>![O=C1[C@](C)(CC(OCC)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O>CC#CO[Li]>[*]](/media/uploads/2a62e453-ef96-4843-ad19-79a3516a6a38.png)</MultiModal>

如上反应产生的产物$\bf{A}$含有三个五元环。该产物的形成依次经过了两个关键的电中性中间体$\bf{B}$和$\bf{C}$。

已知产生$\bf{B}$的同时产生了$\rm{LiOEt}$。

下列说法正确的是：
All other options are incorrect
$\bf{B}$ contains three rings
$\bf{C}$ contains three rings
$\bf{A}$ produces only one small molecule, $\rm{LiOEt}$, simultaneously.
The degree of unsaturation of the product of the reaction of $\bf{A}$ with $\rm{O_3, Zn/MeOH}$ is 6
$\bf{A}$ contains four chiral carbon atoms
其他选项均不正确
$\bf{B}$含有三个环
$\bf{C}$含有三个环
$\bf{A}$产生同时只产生了$\rm{LiOEt}$一种小分子
$\bf{A}$与$\rm{O_3, Zn/MeOH}$反应的产物的不饱和度为6
$\bf{A}$含有四个手性碳原子
Lithium acetylide is similar to an enolate anion and has strong nucleophilicity. Because $\rm{LiOEt}$ is produced, it can be determined that the ethyl ester group in the substrate is nucleophilically attacked by the acetylide anion to form a ketene structure. Therefore, intermediate $\bf{B}$ is O=C1[C@](C)(CC(C(C)=C=O)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O.

<Checkpoint>[Lithium acetylide is similar to an enolate anion and has strong nucleophilicity](1)</Checkpoint>

<Checkpoint>[The ethyl ester group in the substrate is nucleophilically attacked by the acetylide anion to form a ketene structure](1)</Checkpoint>

<Checkpoint>[$\bf{B}$ is O=C1[C@](C)(CC(C(C)=C=O)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O](1)</Checkpoint>

Ketene has strong electrophilicity. According to the hint that product $\bf{A}$ has three five-membered rings, $\bf{B}$ is highly likely to undergo ring formation; considering an intramolecular reaction, the intramolecular carbonyl oxygen atom nucleophilically attacks ketene to form an enolate anion, and the enolate anion then nucleophilically attacks the carbonyl group, which is positively charged at this time, to form a five-membered ring and a four-membered ring structure. Therefore, $\bf{C}$ is O=C1[C@@]2([H])CC(C)(C)C[C@@]2([H])C3(O4)[C@]1(C)CC(C3(C)C4=O)=O.

<Checkpoint>[$\bf{C}$ is O=C1[C@@]2([H])CC(C)(C)C[C@@]2([H])C3(O4)[C@]1(C)CC(C3(C)C4=O)=O](2)</Checkpoint>

According to the structural formula, $\bf{C}$ contains four rings and $\bf{B}$ contains two rings, so options B and C are incorrect.

<Checkpoint>[$\bf{C}$ contains four rings and $\bf{B}$ contains two rings](1)</Checkpoint>

The four-membered fused lactone structure is extremely unstable and easily loses carbon dioxide to form a double bond. Combined with the hint of three five-membered rings, the final product $\bf{A}$ structure is O=C([C@@]1(C)C2)[C@@]3([H])CC(C)(C)C[C@@]3([H])C1=C(C)C2=O. $\bf{A}$ has three chiral carbons, so option F is incorrect.

<Checkpoint>[The four-membered fused lactone structure is extremely unstable and easily loses carbon dioxide to form a double bond](1)</Checkpoint>

<Checkpoint>[$\bf{A}$ structure is O=C([C@@]1(C)C2)[C@@]3([H])CC(C)(C)C[C@@]3([H])C1=C(C)C2=O](1)</Checkpoint>

The reaction produces two small molecules, $\rm{CO_2, LiOEt}$, so option D is incorrect.

<Checkpoint>[The reaction produces two small molecules, $\rm{CO_2, LiOEt}$](1)</Checkpoint>

The final product $\bf{A}$ undergoes ozonolysis, and the five-membered ring opens to give two carbonyl groups. The product contains four carbonyl groups and two five-membered rings, and the degree of unsaturation is 6, so option E is correct.

<Checkpoint>[$\bf{A}$ undergoes ozonolysis, the product unsaturation degree is 6](1)</Checkpoint>

<MultiModal>![$\bf{B}$ is O=C1[C@](C)(CC(C(C)=C=O)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O；$\bf{C}$ is O=C1[C@@]2([H])CC(C)(C)C[C@@]2([H])C3(O4)[C@]1(C)CC(C3(C)C4=O)=O；$\bf{A}$ structure is O=C([C@@]1(C)C2)[C@@]3([H])CC(C)(C)C[C@@]3([H])C1=C(C)C2=O。](/media/uploads/18982801-d129-446a-86d8-9c6ff99b5748.png)</MultiModal>
炔醇锂类似于烯醇负离子，具有强亲核性，由于产生了$\rm{LiOEt}$，所以可以判断底物中乙酯基被炔醇负离子亲核，生成烯酮结构。因此中间体$\bf{B}$为O=C1[C@](C)(CC(C(C)=C=O)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O。

<Checkpoint>[炔醇锂类似于烯醇负离子，具有强亲核性](1)</Checkpoint>

<Checkpoint>[底物中乙酯基被炔醇负离子亲核，生成烯酮结构](1)</Checkpoint>

<Checkpoint>[$\bf{B}$为O=C1[C@](C)(CC(C(C)=C=O)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O](1)</Checkpoint>

烯酮具有强亲电性，根据产物$\bf{A}$有三个五元环的提示，$\bf{B}$很大概率需要成环；考虑分子内反应，分子内羰基氧原子亲核烯酮形成烯醇负离子，烯醇负离子再去亲核此时具有正电性的羰基，可形成五元环并四元环的结构。因此$\bf{C}$为O=C1[C@@]2([H])CC(C)(C)C[C@@]2([H])C3(O4)[C@]1(C)CC(C3(C)C4=O)=O。

<Checkpoint>[$\bf{C}$为O=C1[C@@]2([H])CC(C)(C)C[C@@]2([H])C3(O4)[C@]1(C)CC(C3(C)C4=O)=O](2)</Checkpoint>

根据结构式可知$\bf{C}$含有四个环，$\bf{B}$含有两个环，选项B,C错误。

<Checkpoint>[$\bf{C}$含有四个环，$\bf{B}$含有两个环](1)</Checkpoint>

四元并环内酯结构极其不稳定，容易脱去二氧化碳形成双键，结合三个五元环提示，最终产物$\bf{A}$结构为O=C([C@@]1(C)C2)[C@@]3([H])CC(C)(C)C[C@@]3([H])C1=C(C)C2=O。$\bf{A}$具有三个手性碳，选项F错误。

<Checkpoint>[四元并环内酯结构极其不稳定，容易脱去二氧化碳形成双键](1)</Checkpoint>

<Checkpoint>[$\bf{A}$结构为O=C([C@@]1(C)C2)[C@@]3([H])CC(C)(C)C[C@@]3([H])C1=C(C)C2=O](1)</Checkpoint>

反应产生了$\rm{CO_2, LiOEt}$两种小分子，选项D错误。

<Checkpoint>[反应产生了$\rm{CO_2, LiOEt}$两种小分子](1)</Checkpoint>

最终产物$\bf{A}$发生臭氧化反应，五元环开环得到两个羰基，产物含有四个羰基和两个五元环，不饱和度为6，选项E正确。

<Checkpoint>[$\bf{A}$发生臭氧化反应，产物不饱和度为6](1)</Checkpoint>

<MultiModal>![$\bf{B}$为O=C1[C@](C)(CC(C(C)=C=O)=O)C([C@@]2([H])CC(C)(C)C[C@]21[H])=O；$\bf{C}$为O=C1[C@@]2([H])CC(C)(C)C[C@@]2([H])C3(O4)[C@]1(C)CC(C3(C)C4=O)=O；$\bf{A}$结构为O=C([C@@]1(C)C2)[C@@]3([H])CC(C)(C)C[C@@]3([H])C1=C(C)C2=O。](/media/uploads/18982801-d129-446a-86d8-9c6ff99b5748.png)</MultiModal>