This question does not require stereochemistry. Figure 1 shows a two-step reaction.

<MultiModal>![Fig. 1, the figure shows a two-step sequential reaction. The first step, described by SMILES, is: C=C(C)/C=C\[C@H](C)C[C@H](C1=C[C@@H](CCCC1=O)O[Si](C)(C)C(C)(C)C)O>[A], reaction conditions: o-xylene, 140℃. The second step, described by SMILES, is: [A]>>[B], reaction conditions: PCC (pyridinium chlorochromate), $\mathrm{Al_2O_3}$ (aluminum oxide), DCM (dichloromethane), then TBAF (tetrabutylammonium fluoride), THF (tetrahydrofuran), room temperature.](/media/uploads/47897ac4-d51f-4349-9cce-e868b505e91c.png)</MultiModal>

Deduce the structural formula of $\mathbf{A}$.

In the subsequent transformation of $\mathbf{A}$, an unexpected reaction occurred, yielding compound $\mathbf{B}$. It is known that $\mathbf{B}$ contains only 3 rings, and $\mathbf{B}$ does not contain a hydroxyl group. Deduce the structural formula of $\mathbf{B}$ and the structural formula of the key intermediate in the process from $\mathbf{A}$ to $\mathbf{B}$.

There are the following statements:

1.  $\mathbf{A}$ does not contain a five-membered ring.
2.  $\mathbf{A}$ contains four rings of seven members or less.
3.  A carbon-carbon single bond is formed in the process of generating $\mathbf{B}$.
4.  $\mathbf{B}$ is an α,β-unsaturated ketone.

Among the following options, the one with all correct statements and the most correct statements is:
本题不要求立体化学，图1为两步反应。

<MultiModal>![Fig. 1，图中为两步连续反应，第一步反应以SMILES描述为：C=C(C)/C=C\[C@H](C)C[C@H](C1=C[C@@H](CCCC1=O)O[Si](C)(C)C(C)(C)C)O>[A]，反应条件为邻二甲苯、140℃。第二步以SMILES描述为：[A]>>[B]，反应条件为PCC（吡啶氯铬酸盐）、$\mathrm{Al_2O_3}$（氧化铝）、DCM（二氯甲烷），然后TBAF（四丁基氟化铵）、THF（四氢呋喃）、室温。](/media/uploads/47897ac4-d51f-4349-9cce-e868b505e91c.png)</MultiModal>

推断出 $\mathbf{A}$ 的结构简式。

在$\mathbf{A}$的后续转换中，发生了意料之外的反应, 得到了化合物 $\mathbf{B}$。已知 $\mathbf{B}$ 中只含有 3 个环，且$\mathbf{B}$中不含有羟基。推断$\mathbf{B}$的结构简式,和$\mathbf{A}$到$\mathbf{B}$过程中关键中间体的结构简式。

有以下几种说法：

1. $\mathbf{A}$中不含五元环
2. $\mathbf{A}$含有四个七元及以下的环
3. 生成$\mathbf{B}$的过程中形成了碳碳单键
4. $\mathbf{B}$为α，β-不饱和酮

下列选项中说法全部正确且正确的说法最多的为：
All other options are incorrect
1
2
3
4
1,2
1,3
1,4
2,3
2,4
3,4
1,2,3
1,2,4
2,3,4
1,2,3,4
其他选项均不正确
1
2
3
4
1,2
1,3
1,4
2,3
2,4
3,4
1,2,3
1,2,4
2,3,4
1,2,3,4
The first step, high temperature, is the classical D-A reaction condition. The molecule contains exactly one dienophile and one diene, and the two undergo a D-A reaction to form compound $\mathbf{A}$, as shown in Figure 2. Statement 1 is incorrect, and statement 2 is incorrect.

<MultiModal>![Fig. 2，The molecule in the figure is described by SMILES as:CC1=C[C@@]2(C)[C@H](C)C[C@H]([C@]32C(=O)CCC[C@H]([C@]3(C)C1)O[Si](C)(C)C(C)(C)C)O](/media/uploads/82124102-91e0-4bb8-9191-db8f467d4944.png)</MultiModal>

<Checkpoint>[Intramolecular D-A reaction occurs at high temperature to generate the tricyclic product $\mathbf{A}$](1)</Checkpoint>

<Checkpoint>[$\mathbf{A}$ is described by SMILES as:CC1=C[C@@]2(C)[C@H](C)C[C@H]([C@]32C(=O)CCC[C@H]([C@]3(C)C1)O[Si](C)(C)C(C)(C)C)O](1)</Checkpoint>

In the second step, PCC is a classical hydroxyl oxidizing agent, oxidizing the unprotected hydroxyl group in $\mathbf{A}$ to a carbonyl group. TBAF deprotects the silicon-containing protecting group, forming a hydroxyl anion. Generally, the product can be obtained after workup. However, the question mentions that an unexpected reaction occurred, and the product $\mathbf{B}$ does not contain a hydroxyl group, indicating that the hydroxyl group underwent further reaction. Consider possible reactions: The carbanion generated by the retro-aldol condensation/Michael addition reaction is unstable, which is excluded. It is found that the hydroxyl anion can attack the carbonyl group on the seven-membered ring, forming a stable five-membered and six-membered bridged ring structure. The structure of this intermediate is shown in Figure 3.

<MultiModal>![Fig. 3，The molecule in the figure is represented by SMILES as:CC(C[C@]1([H])[C@@]23[C@]4([O-])CCC[C@H]1O4)=C[C@@]3([H])[C@H](C)CC2=O](/media/uploads/c2a26942-b2c2-4972-961d-30dea86d382b.png)</MultiModal>

<Checkpoint>[After removing the protecting group, the hydroxyl group attacks the carbonyl group on the seven-membered ring to form a bridged ring intermediate.](1)</Checkpoint>

<Checkpoint>[The structure of the intermediate is described by SMILES as:CC(C[C@]1([H])[C@@]23[C@]4([O-])CCC[C@H]1O4)=C[C@@]3([H])[C@H](C)CC2=O](1)</Checkpoint>

At the same time, considering that there are only three rings in $\mathbf{B}$, it indicates that carbon-carbon bond cleavage further occurred, reducing the number of rings. The hydroxyl anion formed by the attacked carbonyl group can further undergo an elimination reaction to generate a stable enolate anion, and product $\mathbf{B}$ is obtained after workup, the structure of which is shown in Figure 4.

<MultiModal>![Fig. 4，CC1=C[C@]2([C@@H](CC([C@]2([C@](C1)([C@@H]3CCCC(O3)=O)[H])[H])=O)C)[H]](/media/uploads/fb90d65c-cfaa-49ad-916d-65d921822f5b.png)</MultiModal>

<Checkpoint>[A retro-aldol condensation reaction occurs to generate a stable enolate anion, and product $\mathbf{B}$ is obtained after workup](1)</Checkpoint>

<Checkpoint>[Product $\mathbf{B}$, described by SMILES as:CC1=C[C@]2([C@@H](CC([C@]2([C@](C1)([C@@H]3CCCC(O3)=O)[H])[H])=O)C)[H]](1)</Checkpoint>

In the process of generating $\mathbf{B}$, a carbon-oxygen double bond and a carbon-oxygen single bond are formed, and a carbon-carbon single bond is broken, but no carbon-carbon single bond is formed, so statement 3 is incorrect. $\mathbf{B}$ does not contain conjugated double bonds, so statement 4 is incorrect. Therefore, all the above statements are incorrect, and the answer is ${A}$
第一步高温为经典D-A反应条件，分子内恰好含一个单烯体和一个双烯体，二者发生D-A反应形成化合物$\mathbf{A}$，如图2。说法1错误，说法2错误。

<MultiModal>![Fig. 2，图中分子以SMILES描述为：CC1=C[C@@]2(C)[C@H](C)C[C@H]([C@]32C(=O)CCC[C@H]([C@]3(C)C1)O[Si](C)(C)C(C)(C)C)O](/media/uploads/82124102-91e0-4bb8-9191-db8f467d4944.png)</MultiModal>

<Checkpoint>[高温下发生分子内D-A反应，生成三环产物$\mathbf{A}$](1)</Checkpoint>

<Checkpoint>[$\mathbf{A}$以SMILES描述为：CC1=C[C@@]2(C)[C@H](C)C[C@H]([C@]32C(=O)CCC[C@H]([C@]3(C)C1)O[Si](C)(C)C(C)(C)C)O](1)</Checkpoint>

第二步PCC为经典羟基氧化剂，将$\mathbf{A}$中未保护羟基氧化为羰基。TBAF对含硅保护机脱保护，形成一个羟基负离子。一般情况下后处理即得到产物。但是题中提到发生了意外的反应，且产物$\mathbf{B}$不含羟基，说明羟基发生了进一步的反应。考虑可能的反应：逆羟醛缩合/Michael加成反应生成的碳负离子不稳定，排除。发现该羟基负离子可以进攻七元环上的羰基，形成一个稳定的五元、六元桥环结构，该中间体结构如图3。

<MultiModal>![Fig. 3，图中分子以SMILES表示为：CC(C[C@]1([H])[C@@]23[C@]4([O-])CCC[C@H]1O4)=C[C@@]3([H])[C@H](C)CC2=O](/media/uploads/c2a26942-b2c2-4972-961d-30dea86d382b.png)</MultiModal>

<Checkpoint>[脱除保护基后，羟基进攻七元环上的羰基，形成桥环中间体。](1)</Checkpoint>

<Checkpoint>[中间体结构以SMILES描述为：CC(C[C@]1([H])[C@@]23[C@]4([O-])CCC[C@H]1O4)=C[C@@]3([H])[C@H](C)CC2=O](1)</Checkpoint>

同时考虑到$\mathbf{B}$中只有三个环，说明进一步发生了碳碳键断裂，减少了环的数量。被进攻的羰基形成的羟基负离子可以进一步发生消除反应，生成一个稳定的烯醇负离子，后处理得到产物$\mathbf{B}$，其结构如图4。

<MultiModal>![Fig. 4，CC1=C[C@]2([C@@H](CC([C@]2([C@](C1)([C@@H]3CCCC(O3)=O)[H])[H])=O)C)[H]](/media/uploads/fb90d65c-cfaa-49ad-916d-65d921822f5b.png)</MultiModal>

<Checkpoint>[发生一步逆羟醛缩合反应，生成稳定的烯醇负离子，后处理得到产物$\mathbf{B}$](1)</Checkpoint>

<Checkpoint>[产物$\mathbf{B}$，以SMILES描述为：CC1=C[C@]2([C@@H](CC([C@]2([C@](C1)([C@@H]3CCCC(O3)=O)[H])[H])=O)C)[H]](1)</Checkpoint>

生成$\mathbf{B}$的过程中形成了碳氧双键、碳氧单键，断裂了碳碳单键，而并没有形成碳碳单键，说法3错误。$\mathbf{B}$不含共轭双键，说法4错误。所以以上说法均错误，答案为${A}$